Thanks David,
I think I've mostly figured this out. TT desktop does allow you to file form 3115, it just won't fill it out for you.
I've entered the result of the 481(a) adjustment on lines 48 and 27b of Sch C (via the relevant TT question about other expenses).
The only thing which is slightly awkward is that 3115 requires attaching an explanation of certain things which can't be done in TT afaik. So something has to be printed out and filed, but the IRS requires submitting 3115 by mail also. It would be great if you or someone else could confirm this was the right approach.
At this point I'm more curious what TT is doing when you tell it you have assets brought into service several years ago for which no prior depreciation has been claimed. It does allow more depreciation but not the amount computed via MACRS. (To be clear, that's not what I'm doing for the filing. I'm telling TT prior depreciation was applied, and then taking the extra deduction via 481(a) on 3115).
