I have heard it's easier to go from Visa-Visa and MC-MC than try and cross over. YMMV of course.
I too would like to PC my Apple card, but a while back I sent them a secure message and they said its a no go. The only way would be to app for the card itself
Usually it's only possible if your current product is End-Of-Life, like the old Juniper cards or one of the airline cards that's doomed due to consoldation in the industry.
They are, however, very easy to work with in getting new cards as long as you're willing to take the inquiry hit and new account on your report. If you apply for the new card and are denied, at your request they will move some (or all) over the limit from your old card to the new one to open it -- if you otherwise don't meet their criteria for additional credit.
Of course, there are always exceptions to every "rule" -- so if you really want to do it, can't really hurt to ask.
Yeah I have not heard of many success stories involving PC with Barclay cards. You'd probably have to just app for the new card.