I think rather than lock old threads, it might be better to highlight the age somehow. Like when I am not logged in, there is a pink banner prompting me to register. Maybe a warning "THIS THREAD IS MORE THAN TWO YEARS OLD" across the top as a reminder to those who may not notice the date and offer a solution to an antique problem.
I just hope the main forum remains quick and simple and the changes are minimal. My internet access at home has to bounce through a wifi booster, off of some trees and to a cell tower 14 miles away. This site is about the only one that works without long delays when the weather is bad.