If you want update historical data while cutover it's difficult else you can load historical data as chunks and then during switchover apply access exclusive lock on table and load latest few hours data and rename table in single transaction.
Is there a way to partition a table in production without downtime? Or extension that allows it.
Thanks!
Zero downtime (or without a table lock at some point) is not possible with native partitioning since you cannot convert an existing table into a natively partitioned table. However, it is possible to do it with very little downtime if the circumstances allow. I've laid out a method of doing this with pg_partman here - https://github.com/pgpartman/pg_partman/blob/master/doc/pg_partman_howto_native.md