Please bear in mind that the v8 output module interface is not stable at this time. It will very likely change within the next weeks.
Right now, the rsyslog v8 compatibility doc has some information on the new interface. It is expected that it will remain the best source to get at least an overview of the required changes.
Actual samples are best found in git. As I have various levels of updates (from very minimal to full), you'll probably find something for your liking. Very minimal change is required for message modification modules (assuming they were thread-safe in the first place), e.g. mmfields:
It's actually just adding the new plumbing. That's all that's usually required for message modification modules. Still, note the fundamental difference that the modules are run concurrently in v8, and as such must be reentrant and thread-safe (this requirement now applies to all output modules). Usually this is the case, but may be different depending on the processing done by the module in question (e.g. lookups to backend systems, libraries, global data - to name some of potential trouble spots).
omrelp is an example of a conversion to v8 that makes use of the new features:
If you browse git, the commit comments should help find more examples on a module-by-module basis.