It really depends how specific whatever software/application is to the unique layout and structure of centos vs debian.

If it's some common LAMP stack thing duplicating exactly the same thing on debian+nginx(or apache2)+php7.3+mariadb could be trivially easy.

For organizations that have things like ansible set up to auto provision things where the configuration files live on a centos system, obviously locations and naming will need to be changed.

there are also certain weird (usually closed source binary) applications shipped by software vendors for specific vertical industries, which ONLY exist in a form for RHEL or CentOS. It'll be interesting to see how that works out. The vendor won't support running it on debian or ubuntu server or anything else.

I would say that out of all the possible distros of Linux, RHEL is by far the one that I see with the most rare, weird, expensive paid software that gets developed for it, due to its extensive use in government and fortune 100 type environments. In which case I don't think much will really change since the same customers will go right on buying RHEL paid yearly licenses.

absolutely huge pain in the ass. The issue isn't typically around installing a new linux distro; that works fine, these days. it's usually getting all your in-house code and processes to work properly with the new environment that's the nightmare. you'll be handling weird bug reports related to the migration for months.

which is in no way redhat's, or centos's fault.

if you have 100 machines you need to install 100 new OS's which just by itself is a bunch of work.

The actual install of linux is generally pretty straight forward but to do it minimum downtime you need spare boxes to build with the new distro (preferably 1 for each old box, but you can get away with less, its just takes more time), install your software on, and then test. You also need sysadmins and testers to do the work, and developers to fix any issues (normally around library versions etc). Then you have the risk that something was missed in the install & testing and causes issues in prod a new distro will not be 100% bug compatible with your old one so you will find problems.

And thats assuming you have the skills to install the new distro, how to set it up as required, how to reinstall the software (its amazing how much in house software doesn't have install docs or even source code), and the skills to run it 24/7. If not factor in training and self-learning time for everyone.

Its all doable, but its probably months of work and a bunch of risk just to get back to where you were on a new distro, which makes no difference to your customers at all.

Theres a reason companies tend not to switch very often.