Best approach to migrate server from droplet to droplet with floating/reserved ip

Hi! First of all I’m an app developer but I’m new to server development. I currently have this app-server configuration working fine:

iOS/Android app (pro) --> mysubdomain1.domain.com --> Digital Ocean Floating IP --> Droplet #1 (which has a Let’s encrypt certificate associated to mysubdomain1.domain.com).

Now I have a new back-end on a Droplet #2 (more resources than Droplet #1, with more features, same region as Droplet #1 so Floating IP can change between them…). This scenario is working fine too:

iOS/Android app (test) --> mysubdomain2.domain.com --> Droplet #2 (which has a Let’s encrypt certificate associated to mysubdomain2.domain.com).

I would like to know the best (and least risky) way to migrate from the first scenario to the second one (I could make a backup and restore of the back-end database without problems in minutes from one droplet to the other). I guess that this will fail:

iOS/Android app (pro) --> mysubdomain1.domain.com --> Digital Ocean Floating IP (pointing now to the new droplet) --> Droplet #2

I guess this will fail because Let’s encrypt certificate in Droplet #2 is for mysubdomain2.domain.com but connection will be made through mysubdomain1.domain.com.

I was thinking on setting Droplet #2 to mysubdomain1.domain.com too before changing the Floating IP. But I guess this will fail again because Let’s encrypt won’t access Droplet #2 (because mysubdomain1.domain.com points to Droplet #1) and Droplet #2 won’t have the Let’s encrypt certificate.

I wonder if it’s possible to copy the Let’s encrypt certificates from Droplet #1 to Droplet #2 so Droplet #2 supports connections from both subdomains.

So, how could I switch from droplet #1 to droplet #2 in the least risky and easiest way? I’ve read about load balancers and such things but I’m afraid it’s too deep for my server knowledge at the moment.