According to Motorola's website, the IMPRES mobile mics you're thinking of are only compatible with the XPR mobiles, as the XTL series palm mics are only compatible with the XTLs (shown in the "Compatible Products" tab of each mic). However, this is most likely a marketing thing - but don't take my word for it, I could be wrong. However, the connectors look identical. Not to mention, the two palm mics with keypads are identical as well but have different compatible models, one (XPR) is IMPRES and the other (XTL/APX) doesn't seem to be. So basically they seem interchangeable, but one set is IMPRES, while the other isn't. I don't know if there's any technological or hidden changes that could prevent them being interchangeable, but there very well could be. Don't take this as an expert opinion, just a peek on the Motorola website.
Little late, but happy Independence Day!
EDIT: W8RMH beat me. But I kind of just gave the justification to what he said, haha.