*Steam/retail install to epic games gtav conversion. ***forewarning - rumored to need a little download before instructions work - I put this together (see orig post/bottom) before guides were 'precise' - start install and wait atleast 1% before continuing. I will not test the validity).

------Edit----- and yes, to confirm - I started the gtav install in epic launcher. Then force closed launcher via task manager (I do recall it going to atleast 1%). Copied all .rpf's and folders from steam (or retail) gtav install over to the new install (in my case it was d\:\GTAV\ (AND THEN within that --- .egstore\bps\Install <--- copy to location)). I kept the existing files. Started epic launcher and resumed installation - it quickly rocketed up to 90% installed. Will need to check files after it is done just to be sure - but it appears possible to prevent a large portion of redownload.

*got to 93% and it moved files to correct location and started verifying automatically.
*it finished, didn't appear to complain about files - cleaned up the .egstore/bps/install folder) and is letting me launch now.

*launching prompts the reinstall of r* launcher, social club, directx, and vcredists.


edit - uninstalled egs gtav/launcher- afterwards reverted r* launcher back to play from steam


Original pondering/untested post -
If the epic games launcher only facilitates an account creation/link with R* - and still uses the R* launcher to startup and go - you most likely can use a large part of the steam gtav install - just like you can use it for the steam install to retail conversion. Really depends on how epic handles installation (if it does, as the r* launcher will detect files), and if there is a verify files function to correct any other issues.

General steps for steam to r* here -
https://psteamcommunity.yuanyoumao.com/app/271590/discussions/0/617335934131510296/#c142260895145976010

Although somewhat changed since new launcher - many have reported simply copying the folders/.rpfs over - and pointing the launcher to that location and having the rest fixed.
YMMV.

https://psteamcommunity.yuanyoumao.com/app/271590/discussions/0/4111168970094086225/#c4111168970094177787