I worked with sites that under huge load and has huge memcached&db setups.
We constantly backup&migrate our databases to keep a good system.
I can create a simple php migration tool for you, that you can chose two tables as start and end point, then links the columns in which you want the data to be transferred. So the system creates the task and a cron would do the job over time. No load danger for your old system. You can change the rate of the transfer.
Also my script could sync the two databases, that means. if you work the script again, it would check for newly added lines and sync the databases again. Allowing for a smooth and datalossless switch between databases