Change Sharepoint 2016 roles from custom to frontend

[Deleted]

I have planned an installation of  a new SP farm, however the customer has only one server available, its a farm for 400 users and maximum 40 users concurrently, so I think with a minrole (custom) it will be enough.

However after a few months we have the requirement to migrate a network drive with about 1TB of information, so we will create a document management portal for that with content types, libraries, subsites, etc.

At that time we wil surely need additional servers, lets say minimum one additional server with the search minrole.

Questions are:

1. Can I change  a server from role custom to Front end? what about if I started search services there? then what would I need to do?

2. The farm wil hold many documents, however the concurrent users are not too many, any recommended nr of servers for 1TB of information so we have a fresh crawl and fast queries?

 Number of files 600,000

Thanks

[#2]

1) Yes, via Central Admin -> System Settings. Juts "fix" the role if it flags the server. It's a one-click operation.
2) I usually recommend 4 servers, 2 DC+FE, 2 App+Search. Might be overkill, but it's a good place to start due to high availability of the solution. Just don't schedule any full crawls; use Continuous Crawl.