Maybe not directly related to your question, but what made you stuck with 5.99 ?
Is it some bug that prevents you from proceeding, or you just need to share your project with 5.1 users?
In case of a bug, proper report on Gitlab will be probably the quickest and simplest solution as the show-stopping bugs are taken very seriously by the developers.
The schematic file format has changed completely between V5.1.x and V5.99.
You can forget about trying to edit it a bit with a text editor.
For changing in other file formats…
A simple way to get some idea is to take a simple V5.1.x project, then open & save a copy of it with V5.99 and then use something like meld merge to look at the differences. It may also change the order in which things are written to the files, which is an extra complication (and git unfriendly), but I believe some development effort is also going that way.
@ cbhirud, have you solved this issue? Personally, I think it is not possible to revert to v5 file format by using text editor, but maybe you succeeded (just curious)?
At least there is no need to redo the PCB layout.
You can open your project in V5.99 and create gerber files from it, and then open those gerber files in Gerbview from KiCad V5.1.9, and then: Gerbview / File / Export to Pcbnew…
A lot of info is lost in gerber files. The concept of footprints for example just does not exist in Gerber format. But what you can recover is:
Pcb Outline, mounting hole locations.
All tracks (Zones probably too).
Footprint locations (Implicitly encoded in the end of tracks. Snap pads to end of tracks in Pcbnew.
Netlist. This is re-created from the existing tracks and the new footprints you put on top of them. I once found 3 errors this way during reverse-engineering a project. You then keep changing (adding to) the schematic and Update PCB from Schematic untill DRC in Pcbnew is happy and shows no more discrepancies.