A bit silly but that is how it works. Because you have just unlocked Hera Tani, you have to build your reputation with her before you are able to upgrade a module to Grade 5. Once you have upgraded a module, like you have done with your powerplant, you can not diwngrade it within that same spec. You have to either choose a different spec at Grade 1, or grind up the engineers rank using a different “throw-away” module.
i take it by now you have already scrapped your previous powerplant mod, but if you run into a similar situation in the future here is a good strategy I’ve used that seems to be the least painful:
Show up at engineer base for first time, observe “hmm i’ve already got a G4 overcharged PP, I want G5, but this stingy engi only grants G1 at the moment.” At this point either find out if the engineer offers upgrades for other modules (Hera Tani offers Distributor mods so you could go that route) or buy a “throw-away” powerplant in her shop.
Next select the throwaway PP for engineering and browse the offered blueprints. Note you are not looking for a blueprint you necessarily find desirable, you are looking for a blueprint where you have the highest amount of materials to apply Grades 1-4. So lets say you have selected a Class 3 powerplant as your throwaway, you see that you have plenty of materials to apply Grades 1-4 Low Emissions. So you would sit there and climb the engineers reputation ranks by mindlessly applying those blueprints to the throwaway pp until you acheive Grade 5 access with Hera Tani.
Then simply head to outfitting, re-equip your old engineered Powerplant, apply G5 overcharged once, apply an experimental effect if possible, and pin the blueprint for future maxing at your home station.
