I've seen this question about making the same change (50/34 to 50/39) on a Shimano Sora chainset, but I am asking specifically about Ultegra R8000.
I have a bike with Ultegra R8070 (Di2, hydraulic disc brakes, 34/50, 11-28T). I find I am often in the 34-13, and needing a slightly harder gear, but as Di2 doesn't allow the 34-12 combination, I then have to switch chainrings and run up the cassette, only to do the reverse as the gradient/wind changes.
I don't want to change the cassette (as suggested in a comment on the Sora question) as my issue is with the small-sprocket end of the cassette, and I don't want to mess with the gears when in the big ring.
I realise I will have a bigger low gear after doing this (although I could replace the 11-28T cassette with 11-30T).
Is it possible to replace the inner 110mm BCD 34T-MS with a 110mm BCD 39T-MW?
In the dealer's manual for the Ultegra FC-R8000 chainset, Shimano says
Be sure to use the specified gear tooth combination. If an unspecified gear is used, the chain may fall in between the gears, damaging them.
... and then 39T-MW is only allowed with 53T-MW, and 34T-MS with 50T-MS.
Has anyone tried this? Is it okay to replace a 34T inner ring with a 39T one or not?