How to migrate wallet from namecoin to namecore?
Meanwhile I've answered this question at reddit.

You have already taken the first 2 out of 3 steps necessary for your wallet migration: (1) cleaned up the data directory that contained old incompatible files, (2) dumped and imported the private keys from the old wallet (alternatively, depending on how old the wallet is, one could try starting the new namecore client with the -upgradewallet parameter).
The internal bookkeeping hasn't had a chance to detect the old transactions that belong to the newly imported private keys, yet. This is the missing third step: (3) Restart namecore (i.e. namecoind) with the -rescan parameter.

In short: Now you need to restart the new client with the -rescan parameter.
EDIT: Generally I recommend using namecore's new -upgradewallet parameter:
namecoind -upgradewallet -rescan -reindex
as the dumpprivkey/importprivkey approach always bears the risk of confusing the relevant name_update transactions and addresses with older ones. If you do not know how to use the (deprecated) name_debug1 command in Namecoin-Qt then you most likely get it wrong.