I saved my customized client letter as an .rtf (rich text file) and then every year,

delete the content of the current letter via edit function (cut and then paste the content of the saved file. You have to be in a client file to access tools/manage letters/standard letter etc. 

Or simpler you can replace the file itself in the Common folder, fdiltr.rtf for the individual 1040, for example with your identically named file.  (don't remember if there was a problem that caused me to start the cut/paste instead.)

What works for me, I keep many of the data and trigger values which most are not year-specific, so my customizing is ready-made.  You can also change it up a little if you wanted with any new letter codes that might be relevant.

As for the invoice, not the way ProSeries changes around the list of line charges...enough problems when they do it midseason!  Tip: if you have custom line charges at the bottom, put a few extra dummy lines so any midseason additions by PS doesn't overwrite yours.