Do the new TRID rules apply to an assumption of mortgage?

Generally speaking, yes. TRID rules apply to an assumption provided the following three elements are present: 1) it is a residential mortgage; 2) the creditor expresses their acceptance of the new consumer; and 3) there is a written agreement (See 12 CFR 1026.20(b) and associated commentary).

Noteworthy is that the approval of creditworthiness, notification of change of records, mailing of a new coupon book, and acceptance of payments from the new consumer do not in and of themselves constitute the express agreement to accept the new consumer as required.