If you like drama, such items could stay altered, until reunited with the original target, dispelled or the duration of the spell expires.
If for any reason a magic item wont, cant or can resist being altered, an RR would be appropriate, with failure indicating it does change for the duration of the spell. I have often required an RR for artifact powered gear, with the items rr success being described as "it wavers and shimmers...or...you twist the items shape, almost completeing the transformation, but it is to powerful a stain on you, and you cant finish the weave..."
Items like these can be a source of fun, adventures, and considering the power the lend to PC's, suffering some small disadd doesn't strike me as a bad thing.
If you go with the item staying altered when seperated, remember a few things. The spell caster will be able to cancel the spell just by concentrating, causing the altered item to assume normal form. Also, detects should show an active spell, and the right detect what spell (i.e. change to kind, etc).
Perhaps the best bet it to assume that any item more than 100' away from the reciepient of the changing magic reverts to its original form. Also recall changes should be cosmetic; short sword into dagger, bracer into bracelet, etc.
lynn