Currently we are migrating from Marklogic version 8 to Marklogic version 10.
We can do replication between forests of same version Marklogic-v10 to Marklogic-v10, so i need to understand whether we can have replication between forests of ML-v8 to ML-v10 and if we can, what problems can arise if we try to do that?
Flexible Replication works between MarkLogic 8 and MarkLogic 10, Database Replication as far as I know not. Where Database Replication operates on a fairly low level (it sends journal frames, and forest data across) with a configurable allowed Replication Lag, Flexible Replication operates on fairly highly level of abstraction, and uses asynchronous communication by design (master does not wait for its completion).
Database Replication allows for a certain level of consistency. Flexible Replication however can be used for instance in master-master scenarios. Each has its own use cases, but if you need to maintain different major versions of MarkLogic, Flexible Replication is the only option of the two.
Next to that, there are also ways to move data between clusters outside of MarkLogic, using tools like MLCP, Corb, or NiFi for instance. But those are usually more suited for one-time migrations in such cases.
Please note that MarkLogic 8 has reached End of Life: https://help.marklogic.com/
HTH!