Just take it slow and methodical.
Try to understand EVERYTHING before you attempt the mod.
Learn to understand what the circuit does originally and WHY you are modifying it, then understand HOW the modification actually effects the circuit.
When you know this, then the only thing you have to worry about is the actual work done to accomplish the mod.
Again, take it slow and methodical, and Double Check everything.
If there are multiple boards to mod, only do one at a time and put everything else back together for some testing to be sure that there is nothing wrong.
Doing this saves loads of time trying to troubleshoot where the problem is...if there is a problem.
Good Luck!!!