If we currently have the live solochain and in future we need to go on parachain how we will migrate the data from the existing solochain to parachain.
There is a newly released video by parity on launching a Parachain. It might help you.
Check the answer to this previous question on StackExchange: Do not want to clean up the data of the independent chain, how to seamlessly connect to the parachain?.
I like to share too this video of the Sub0 conference: Sub0 Online: Storage Migration - Standalone chain to Parachain
Also is not about migrate data but worth to check this tutorial in Substrate docs about how to Convert a solo chain into a parachain.