Make it a checkbox that I doesn't contain that to list in that sub category like with GAR.
I like that: it's not a refined solution, but it's a start, and I can do it
now. Actually, I did it before typing this

In behaviour is similar to "everyone start as an Amateur, until the choose to become a Pro"; what is missing is the "one-way" constraint (here you can go back), and the enforcement to keep Pro pipsqueaks out of the Amateur scoreboard.
But that can come later. Let's keep this going for the next three months, and see how people feel about it. We'll continue to think about improvements in parallel.
Note:
- this is per-Replay, and not per-Race; one could submit both an Amateur and a Pro replays
- there is nothing stopping a Pro pipsqueak to get into the Amateur scoreboard, and the border between the two groups is blurry; there will be complaints, we'll have to make do.
I'll need to make it
very easy for admins to remove replay tags though

In the meanwhile, it would be very helpful if someone wrote a news post for the homepage, explaining what we are adding, how it works, caveat, and "we reserve the right to remove pipsqueaks from the Amateur scoreboard at our discretion", but in a friendly way.
[edit] let's say we'll consider anyone who has ever placed 6th or higher in a race a Pro.
At least, not limited to.