I just copy mine from my prior year's ProSeries each year. But I use the ProSeries codes, etc extensively.

I just open up the prior year's standard letter( which is basically my perfected client letter, that I save as the Standard Letter each year).  I drag the mouse over the whole letter...……..hit the copy icon (yes in the prior year)...open up the Standard letter in the current year....go to the bottom of the letter as a precaution...and hit the paste icon.

then review..and if my pasted part works fine...i just delete the standard letter above that comes with the program. and save as my standard letter.  Then use the Custom Letter feature to print in each individual return. (which is different this year for sure)