All inner parts are identical except the bottom inner assembly which is slightly smaller in the ultra compact cases – I suggest getting the original case keyboard and just ordering a spare ultra compact bottom inner assembly for your future project (to order this part, add 25 units of the store item “other customizations in $1 increments” to your cart and note it is for an ultra compact bottom inner assembly).

To clarify: if one orders the regular version and the ultra-compact they can not switch between the two?

Yes you can switch between the original style case and the ultra compact case because the parts are interchangeable.  The only different inner part is the bottom inner assembly which is smaller for the ultra compact case version.  If you order a spare ultra compact case and an original style keyboard you will get this smaller spare bottom inner assembly plate.