Recommendation letters post master's

I’m presently in a master’s program. The program is much more relevant to the area I wish to do a PhD in (I switched majors from Bachelor’s->Master’s). I took two recommendation letters from my undergraduate institution to apply for master’s, but the strong one was about a relatively unrelated subfield and I have not kept touch with the professor in question due to the switch. I’ll be applying 3 years post undergrad. In this situation, can I forego the folklore rule of 1 letter of recommendation from an undergrad university ?