Ultimately, we won't know until New Light launches. Additionally, I don't think Bungie would support such an action as it would be outside of their intended installation path so don't expect a response on that. There were some other discussions from people who played Warframe that something like this happened to them. Their platform shifted to Steam and they tried to move their files into place and then performed a verify files. It was a mess and didn't work for them. The differences in the files between Battle.net and Steam are going to be pretty large, and not to mention that what we have now is not what we will have on Steam. When we go to Steam, we will be downloading all the program files that will work with Steam instead of Battle.net. Additionally, we will be downloading Shadowkeep on top of that as well (whether you bought it or not). My hope is that we will be getting the ability to pre-load everything. I'd like to spend the first day playing the game, not downloading it.