Not so difficult but the entire unit really needs to be removed to install the new sidelites properly. It may just be a job that a contractor will not or does not want to do.

"tearing this out would creat thousands of dollars in other repairs."

The unit has to be removed to replace the side lights. In other words, the door cannot be left in place to replace the side lights.

So, basically, yes, it is that difficult.

If the assembly was installed as a one piece framed unit, there are many considerations that have to be addressed when modifying the unit.

Door installers will probably not want to do a job like that. Too many chances for later problems.

Best chance is to find an experienced handy person----but, there are not many folks with that kind of experience simply because there is not much demand for that kind of work---so not much chance for getting experience.

And, because of the probability of a later problem and the lack of warranty, you may wind up with a much worse problem in the future.

Are you replacing the GLASS in the sidelights or the entire sidelights? We had just the glass replaced because I hated the ones that came with the house. (I'm in Michigan btw.)