Did you have to change the format of lha archives to implement the fix?
No.
How did you fix it?
There was a bogus comparison against the datestamp year being greater than 2010. I can only presume the original author meant 2100. Anyhow, now it correctly compares against 2107 which is the last year the date format used can present.
What year will the next lha bug strike?
2108 the latest.