What bugs/issues did you encounter: 

What is your FM version: 18.1.3 1045020

What is your FMRTE version: 18

Which operating system are you using: Windows 10 Professional

As can be seen from the image, I swicthed out a player's tight calf injury to a very serious injury (damaged cruciate ligaments - potentially career threatening).  Though the game shows this up, it doesn't appear to work as his injury still took the same time (a few days) rather than the 180+ days for the injury that he was switched to.

 

I also tried it with another player, with a different type of injury (achilles), with the same issue, being out for 3 days instead of 104-130:

Attachments:  

Upon further testing, it looks like if you use the + feature or FMRTE to add an injury it seems to stay, but it is the editing of one injury to another which does not.

 

Hope that helps.

Thanks for reporting this, but I cannot replicate this issue.

Increasing the Minimum Days and Extra days, doesn't work for you?

 

If it doesn't, please provide us a savegame, and step-by-step instructions about how to reproduce this issue

 

Thanks