no idea if you got your answer… or even if i got the answer.
This is what i got out of reading about it and trying it myself:
All WC3 players now need a (This link is blocked by Blizzard) account.
( had (this link is blocked by Blizzard) installed because i play(ed) WoW.
What i got out of it was that i uninstalled the old Wc3 and then i got the option to install the new one from the new downloadable installer. When i started it i could link my classic account to the reforged one. After that i now can switch between them.