I think that the replies in this post will be helpful: Should I see replies to Mastodon posts that were cross-posted?
If you want to see replies in Micro.blog, you’d want to move your followers from your old Mastodon account to your Micro.blog account. But I don’t think that’s what you want.
If you want your Micro.blog content to be sent to Mastodon, you’ll want to use cross-posting.
Mastodon users can reply to your Micro.blog account directly and you would view their comments on your timeline on Micro.blog only if they subscribe to your Micro.blog AcitivityPub account/stream-- which is sort of like moving instances in Mastodon parlance. Your existing Mastodon account is already on the Social.Tchncs.de instance, and you cannot use Micro.blog to “take over” managing for that instance.
There’s more details in the other thread, but the short version is you can copy posts from Micro.blog to a Mastodon instance automatically (cross posting) and/or you can treat your Micro.blog account as a subscribe-able account from other services that use ActivityPub and it will interoperate directly (which is kind of like Micro.blog becoming your Mastodon instance, just like you could move from Social.tchncs.de).
And just like moving instances within Mastodon, account migration works with some funky caveats.