Can I change my recommendation letter provider? I wrote a few weeks ago about a conundrum I had with a professor regarding my recommendation letter for graduate studies in UIUC. I'm very grateful for everyone's responses, but I still have not received an LOR from this professor. My advisor has asked me to speak to him again (my advisor has spoken on my behalf as well), but I'm beginning to wonder if it's worth it.

Things may not just be uncomfortable with this professor due to our previous interaction, but he also seems to be caught in some private matters, and has had to travel on short notice to his hometown. On top of that, UIUC is a competitive school, so should I just change my recommendation provider? Is it advisable to do so? The professor I have in mind is fantastic, and I've enjoyed her courses and stayed in touch with her, but her field is non-technical. I already have recommendations from great professors in my field, though, and wouldn't it reflect well on my application to be recommended by a professor outside my field as well?

Any advice is welcome. I'm not sure if I'm overthinking this, but I've never heard of anyone changing their recommendation provider AFTER the application has been submitted.
Contact the admissions department, explain that the professor is "caught in some private matters" and hasn't been able to do the letter as a result.  Ask to switch the reference to a different prof.  

In my experience, you're much better off being certain that you don't miss the admissions timelines due to a delayed reference letter than the source of the letter mattering very much.  That being said, I'm not familiar with the specifics of UIUC, so my experience may not be relevant.