Can I upgrade from krc2 to krc4?

KRC2 and KRC4 have completely different architecture so upgrade is not possible.
You may replace entire cabinet and RDC and cables if .... KRC4 has MADA for that robot arm but even so, that is pretty ridiculous approach since it would cost as much as getting new robot with KRC4 cabinet.

Like Panic says, the jump from the KRC2 to the KRC4 was a major technology change, much larger than that from the KRC1 to the KRC2. Off the top of my head, I can't think of a single component that's common between the two model cabinets. Even the cabinet shells aren't compatible; even if you replaced 100% of a KRC2's internal components with KRC4 components (which would be much more expensive than buying a whole KRC4 brand new), I don't think the cooling fans and ballast resistors are compatible.

If you have a KRC2-generation robot, it's usually possible to upgrade it to run from a KRC4 cabinet without a a huge expense, by replacing the RDC/RDW card.

... and cable set and teach pendant

Were you successful with this?

If your question is if this is possible at all - basically if anyone did it - yes it is possible and was done. For example KUKA College in Michigan had several cells with both controllers (KRC2ed05 and KRC4). They would switch to one or another for each class depending on which controller was needed..

In trying to convert the program from one to the other did the syntax change? Has anybody come up with a migrating tool?

KRL syntax did not change. most programs will work without any problem. issues can happen if program is accessing one of few variables that got renamed or if trying to use one of tech packages that went through same evolution cycle... There is a migration document from KUKA on this. not sure why one would spend time to develop tool for things like this as this is rarely used and anyone familiar with KRL can quickly adapt program if needed.

you can try OrangeEdit and play with inline forms from different KSS versions and see what the differences are.

Off the top of my head KSS 8:

1. requires that the :IN and :OUT for variables in the DEF lines of Routines and Functions be user. Previously, it was implicit
2. $POS_ACT deprecated in favor of the new $POS_ACT_MES. Likewise, $AXIS_ACT_MEAS has been added.
3. $PERI_RDY behaves a bit differently
4. Multi-Submit is now supported, and many of the related $PRO_ and $WAIT variables were removed, replaced by the $PROG_INFO array. The CWRITE commands to control different interpreters were also altered.
5. Kdoes not support the old user-messaging system calls (MSG_T), but instead uses the newer KRLMSG tools that were introduced in the late versions of KSS 5.

I'm sure there are a few more, but in general, importing KSS 5.x programs into KSS 8.x, most of the time I only have to fix item 1. Programs that trip over items 4&5 are rather rare, and item 2 doesn't cause any compilation issues, but it's a good idea to switch over to the _MES/_MEAS variables if you were previosly using the original versions.