If you wanted to switch to HR+ in an erg workout, you would then need to set the HR target for that interval in the ride. You would have to do that for every interval, in real time, during the workout.
Most people use HR+ for steady state workouts. They hit quickstart, hit the HR+ button, set their target bpm and then just pedal.
There aren’t many HR+ workouts in the catalogue.
Partly I think that is because each person needs a specific bpm target, rather than a % target.
The workout creator is quick and easy to use and I would say you can convert most workouts that you like the look of to a HR+ near equivalent.
There is more info here
https://forums.trainerday.com/t/hr-controlled-workouts-theyre-great