what is the process to move indexes from one splunk deployment controlled by deployment server A to other splunk deployment controlled by deployment server B
Indexes don't particularly care what DS they're created from. They just exist. Granted, they may have been created from the configurations provided by a particular DS, but that's just configurations.
What I'd suggest is to review your serverclass.conf and indexes.conf files in your deployment apps folders on the original DS and find all the serverclasses that deploy indexes to your indexers. Then copy the stanza(s) from the serverclasses and the indexes from indexes.conf to the new DS, then restart it and you should be good.