Releasing a Vow for a Woolen Tallit Katan

This concerns an Ashkenazi Jew who always says "bli neder" (without a vow) when starting an action he hasn't done before, but this time he doesn't remember exactly if he said it when he decided to try wearing a woolen tallit katan according to the Beit Yosef. Now he wants to return to his previous custom (not wool, but according to the Rema it's according to the Torah requirements). Is a release from the vow necessary? Or is it not considered a matter of custom because it's a clear halacha? Or should we not worry that he forgot to say "without a vow" as he usually does at the beginning?

Shalom u'vracha 

There is no concern of a vow since he did not intend to do it regularly.