There is an option to "import" the old wallet in KWalletManager under File > Import Wallet. I found that my old wallet password was not accepted. Examination of the old folder revealed a salt file. I suspect (and further experimentation supports) that the kwalletd process has one salt file loaded and this one was not being read.
I was able to replace the new wallet file with the old one by replacing my new kwallet directory with my old one:
- Turn off kwallet in
System Settings > Account Details > KDE Wallet > Wallet Preferences > [Uncheck] "Enable the KDE wallet subsystem"
$HOME/.kde/share/apps/kwallet and place your old wallet folder there
- Enable the wallet subsystem (reverse of 1.)
If you need to "merge" two wallets and the import option is not working there are also "Export as XML" and "Import XML" options. You may export from one, switch wallets as above, then import. Be very careful about passwords being written unencrypted to disk! Once you have completed the import run the following command to securely overwrite and delete the XML file:
shred -u /path/to/exported/file
Note: overwriting files is not considered secure deletion on solid state storage media (flash drives, SSDs, etc) as wear leveling prevents exact blocks being written to. Instead, taking care to e.g. turn off swap and mount a RAM disk to write the XML file to are preferable, but that is out of the scope of this answer.