I'm not sure why you would want to convert the weapon tables and armors. Use the ones from the edition you are playing in.
You don't between RMSS and RMU. That's the point. I can mostly use RM2/RMSS ones interchangeably. I can't do that with RMU. I probably wouldn't use the RMU materials in an RM2 or RMSS game as a result. I would say I'd look into if RMU tables could work due to being expanded, however there is far less selection and, more importantly I despise how the size rules work which impacts weapons.
The power level differences between RMSS and RM2 even at 1st level are so extreme that mixing character generation systems is not going to work. Pick one system to build characters.
Why on
Earth would I try mix characters built using RM2 and RMSS? No offense, but that's a huge 'duh'.
You can easily take skills from one edition to another, though. If taking skills from previous editions into RMU or RMSS, you just have to assign them to a category. Going into RM2 you need to assign costs.
Right, as I said, not a big deal to convert, just add or remove category assignment if I want to move from a category system to an individual one. Skill costs and DP provided are the bigger issue.
Taking professions into RMU, you would essentially just design a new profession based on the original one, but at least there is a system for doing so. When converting a profession from RM2 to RMSS, you likewise have to come up with all the costs and details, but there was very little guidance.
Exactly, I would have to redesign RMSS professions to use in RMU. I've already done much of this from RM2 to RMSS, but in most cases I just took a RMSS profession and tweaked it a bit. If RMU came out with professions I didn't already have they could be useful. If they had spell lists I didn't need to convert, more so. The first is unlikely for many years. The second is a distant possibility. But how many spell lists will be in the base RMU that I can't already find in RMSS and RMU? How long before new ones come out that aren't based on those? How many of those won't have spells that hinge on RMU mechanics that I need to tweak?
Creatures and Monsters has a greater mismatch between levels and abilities than RMU's CrL, the difference is that the stats are just made up, it's not as apparent. I mean, would you rather fight a gray gratar, greater giant skeleton, or thauredhel? Do you suppose those are balanced against each other? All 12th level.
I appreciate the time that went into the RMU Creatures and Monsters, but I think that book is a mess in terms of how it seems to have been forced into the design theory of everything else. Talents and flaw costs seemed very inconsistent (very similar talents/flaws had very different costs) and I suspect were often eye-balled in order to make the creature fit the level (talents/flaws seem to basically have been made up to fit the DP requirement for the desired creature level) and not the other way around. The formatting of those also results in too much page flipping and I don't see that changing since that would result in repeat information in an already massive book (even if split). They (everything) have levels, but often times those levels are partially based on DP they have not spent, which means they aren't necessarily really that level until you finish them... which seems to defeat the purpose. I won't even get into the size rules, we'd need a page for that. In a nut shell, I think someone did a lot of nice work on the book with what they were given, but what they were given was a single shaped hole that they had to force all kinds of various shaped creatures through.
With regard to the spell lists, healing is an issue, but those lists exist in previous editions so the need to convert them is questionable. The number of spell lists that actually depend on the combat round is minimal, and that issue likewise existed going between RM2 and RMSS. The majority of spell lists will translate with no issues.
There are multiple factors when trying to convert spell lists because spells can impact so much of the mechanics in the system. They can effect all the various skills, weapons/armor/combat, healing, etc. As an overall piece of RM the spells have a hand in everything. RM has always been a complex system where messing with one piece of it often has an impact on other parts. RMU has changed character build, weapon and armor balance, skills/talents/flaws, round structure, movement, size rules, spell effects, etc.
Stop and look at it this way: Although it's better than Beta 1, I still don't like the round (actions vs movement is still too wonky). I don't like the way creatures are built with talents/flaws I have to look up and are not just listed with them and that they aren't finished (left over DP). I don't like the size rules. None of those things are easily removed/replaced due to the interconnected balance of things. So if I continue to use RMSS they would hope I would still buy RMU materials to back-covert. So, what do I pull from older RM's? Weapon tables, crit tables, spell lists, maybe creatures. Can't use RMU Weapons. Last time I looked I didn't like the RMU crit tables, they are unlikely to come out with ones I don't already have for a long time and even then they SHOULD now be balanced to different weapon tables and armors. Creatures are a mess. Spell lists can be moved back IF there are new ones and IF they aren't tied to things like round mechanics, healing, haste, weapon/armor balance, etc. I like that they are filled in, but it doesn't do me much good if I've already done this myself with essentially the same list from older RM's.
If you were trying to convert characters from RM2 to RMSS or something like that, they yeah, I see where you would have some problems.
And if you're trying to retain your existing fan base...? You're counting on them largely tossing everything they've already done.