What is best way to move large number of existing indexes into new file-group - DB Size: 700GB
The answer is: you cannot run heavy scripts and not slow down the DB server.

I quess all you have to do is to pick kind of light-workload-hours and REBUILD indexes one by one with your own hands monitoring blocking and overall server load.

General advice would be to use

ALTER INDEX &lt;IndexName&gt; REBUILD;

instead of recreating them.

Also consider ALTER INDEX options here such as:


ONLINE
RESUMABLE
MAX_DURATION
MAXDOP
WAIT_AT_LOW_PRIORITY


Have a look at these useful links:


one
two


Take care about free space when rebuilding online, as stated here.