As far as the *possibility* of converting from H-4 to F-1 is concerned - yes, one can switch from H-4 to F-1.
However, you must bear in mind that the F-1 visa is **strictly** a **non-immigrant** visa and you will have to convince the consular officer that you **DON'T INTEND TO IMMIGRATE**.
Since you already have an immigrant petition (I-140), it is fraught with difficulty
and has a high chance of rejection.
In any case, it won't jeopardize your I-140 application if the F-1 is rejected (assuming that you have maintained valid legal status).
On the other hand if F-1 is approved, better not leave the country untill the green card is approved (because you risk re-entry at the POE - because of conflicting dual intents).
