Feature request - Convert PR to `--cherry-pick` (targeting default branch) after it's created

[pedro-procreate]

It'd be great if a PR could be converted to target the default branch after being created.

e.g.:

1. Change is created with spr diff
2. Dependant changes are merged
3. Allow it to be updated into --cherry-pick

[pedro-procreate]

zx script to roughly do it https://gist.github.com/pedro-procreate/143267ec906df99f30ca0b46e51359d3