21.02 introduced dsa switch support on some routers. I’ve upgraded mine, but reverted quickly to 19.07 after I discovered it broke my tinc setup and I couldn’t get it working again, which leads me to 2 questions:
No, you can't.
The decision between swconfig and dsa is made on the source level and involves quite a bit of changes to the individual DTS, network setup scripts and kernel drivers (patches, bare metal development). swconfig support for the migrated targets and kernel 5.4 no longer exists, while it's not inconceivable to resurrect this (given the upstream status of dsa vs rejected out-of-tree status for swconfig - and the addition of new devices that never were supported by swconfig (respectively might not even be supportable by swconfig), it's relatively safe to assume that such changes wouldn't be accepted for migrated targets anyways), it's not a GUI- nor even buildtime 'switch' to toggle.