Hello again,
I think that training a new tokenizer would require training from scratch.   As I said, I’m not an expert, but I would imagine you don’t need to train from scratch.   You could have a look at what the tokenizer is actually doing with the Brazilian Portuguese words (how it is splitting them), and see if that looks reasonable.   Maybe compare it with how it splits some standard Portuguese words.
Running validation takes time and memory, so you could run it only when necessary.