I have two TTs (one for the good lady)
I changed the two lines on the XL and everything worked perfectly, with a slight deviation - see below
I made the same change on the Go530 and the rewritten ttgo.bif just reverts back to the original version (8351) with no ttgo.ori. The saved ttgo.bak shows the 9400 lines
Any words of wisdom would be appreciated.

FYI - It was always my understanding that if you edit a file and save it with a different name, the original file remains unmodified.
This was case when I made the changes - so the statement "At this point, there is only a ttgo.bak file in the root, no ttgo.bif" is not true, and I deleted the original ttgo.bif just to be sure.