How to internally change username in order to not break programs

Hello, I want to know if I can completely change my username internally (so other programs recognize the change) as well as externally (the folder name changes).

What I mean is that my user right now has the address "C:\Users\Firstname Lastname" with the space in between the names. However for a program I am using, it doesn't want spaces in the address. I want it to be "C:\Users\FirstnameLastname". I created the name before I new having spaces can sometimes create issues

I am guessing just renaming the folder as you would with a regular folder (right click, rename) will cause problems. Is there a way I can do it so that it doesn't break all my applications, programs, saved files, etc when the path name changes?

I would prefer if I don't have to create a whole new user as this is my main/local personal account and I have all my stuff saved under this one. But if I must, how can I move everything to the new user? I would make the new one my main.

If I haven't done a good job explaining my question, let me know and I'll try to clear up any confusions. If this is the wrong sub for this, please direct me to the right one.