I put up a WIP to add fundraising/REL1_39 config to jjb/wm-fundraising.yaml and jjb/php.yaml, but it needs some more work to actually run the right tests.
https://gerrit.wikimedia.org/r/876010
In zuul/layout.yaml the fundraising specific gate and test suites are defined to use php73 images. I'd love any suggestions or patch edits from the release engineering team to let us run tests against fundraising/REL1_39 with the php7.4 images while still using php73 (for the next week or so) on the fundraising/REL1_35 branch.
For the last upgrade we needed to maintain a special branch of DonationInterface during the transition, but for this upgrade it seems only one patch is necessary, and that one is backwards compatible with REL1_35.