Regex - replace only last part of an expression
I'm attempting to find the best methodology for finding a specific pattern and then 
replace the ending portion of the pattern.  Here is a quick example (in C#):
//Find any year value starting with a bracket or underscore
string patternToFind = "[[_]2007";
Regex yearFind = new Regex(patternToFind);
//I want to change any of these values to x2008 where x is the bracket or underscore originally in the text.  I was trying to use Regex.Replace(), but cannot figure out if it can be applied.  
If all else fails, I can find Matches using the MatchCollection and then switch out the 2007 value with 2008; however, I'm hoping for something more elegant
MatchCollections matches = yearFind.Matches(" 2007 [2007 _2007");
foreach (Match match in matches){
  //use match to find and replace value
}