Back few years ago after MS purchased Skype they did add support for MSN/Live accounts in that messenger and ultimately, at some point user was forced to log into Skype with MSN account and merge contact lists.

But it seems even today it's still possible to use Skype with old native accounts, along with Microsoft account and what I just discover... with Github accounts. So if they do purchase Discord it might be possible that in future this IM will get support for MS accounts (I wouldn't be surprised if also Github ones) and users will be also offered merge and login with MS account alone containing "old" Discord accounts data for MS definition of "convenience". After all Microsoft account it's their single sign-on feature.

If purchase will take place, I think the question is what they'll do with it: keeping Discord as exclusively game-oriented IM that will somehow be merged with their Xbox infrastructure or abandon Microsoft Teams/merge it with Discord and keep it as business/professional oriented communication platform. Discord might be left alone for a while but I think MS will eventually give it a special place somewhere under their umbrella of services.