We know that it's possible for Yeerks to infest individuals while in morph. In The Hork Bajir Chronicles, a Yeerk called Esplin manages to temporarily infest an Andalite while the Andalite is in morph:
I began to release my hold on the Hork-Bajir brain of my host body. I slithered out, pressing myself down to move quickly out of the Hork-Bajir ear. For a horrible long moment I was blind, connected to neither host.
But then I sensed the new Hork-Bajir ear, the one that was only a morph of the Andalite inside. I squeezed through. I reached desperately with my palps, reaching for contact.
I was still hanging half out of the Andalite's ear when I touched her brain and felt her mind. It was a shock. There it was, a Hork-Bajir brain physically, but within it was not the idiot Hork- Bajir mind, but the lightning-fast Andalite intelligence.
I saw inside the mind, the memories of Aldrea, the Andalite. I saw it all in a flash! All that she had been, all that she had done to thwart us. I saw the secret of the canister.
But most of all, I saw her running, tail high, four eyes open, seeing in all directions at once. Running free across the grass of the Andalite home. 111 I cried, sensing that she was aware of me in her mind.
Esplin was so eager to infest the Andalite that he lost all sense of reason. It had long been his dream to infest an Andalite. Seerowpedia says:
Through this initial research, Esplin fostered a deep desire to one day infest an Andalite, so that he might truly understand them.
Esplin was so overly eager that he forgot the fact that his present Hork-Bajir host would be freed the moment he left him. So, while he was mid-way between the two ears, the Hork-Bajir used his newly reacquired free will to pull Esplin from Aldrea's ear.
Now, Esplin clearly wanted an Andalite (morph-capable) body, not just an Andalite nothlit. So he was probably intending to demorph after infesting her. However, being in the overly eager state he was in, he might not have thought this fully through.
Given what we know about the dynamics of morphing, and other things we may know, what would have happened if Esplin would have fully infested Aldrea, and then made her body demorph to Andalite? Would it have been safe for him to do so?
I obviously know that it works fine the other way around (infest the normal body, then morph). But I'm not sure if it works the other way around. Is there any example of this happening in canon?