Dear Sotar,
Greetings! Thank you for posting to Microsoft Community.
Now, for sure, I am getting afraid about your requirement!
You mean for example, if you have a choice question like this below, you want to change it into text format?

If this is what you mean, I am afraid to say that this is not possible as of now. The only possible way available is to either delete the question and then add the question type of your choice.
By the way, if you wanted to replace a choice question type to a text type, you could delete that, add a new question of your choice and use arrows shown in the screenshot below to change the position of the question. And if this is irrelevant, please ignore:

But it can be possible if you present this idea to Microsoft Form Developers. Infact, please submit your feedback via this link: Microsoft Forms · Community.
Our Product Team is in-charge of the site and they are constantly checking the comments and feedback from our customers.
If I misinterpreted your question, please feel free to post back and correct me.
Thank you for your cooperation. Hope you have a nice day and stay safe!
Sincerely,
Edwine | Microsoft Community Moderator.