You can use VS 2022 to open older NET Framework projects. In general there are no changes that it'll actually make unless the solution was created in a really, really old version of Visual Studio. Irrelevant it will just load them up.
Now, if you attempt to load a project that requires a framework that is not currently installed (older or newer) then VS will pop up a message with several options. The most likely option is to auto-install the missing framework. If you don't want to upgrade anything then this is the way to go. However anything before NET 4 isn't supported at all so that won't be an option anyway. The other option is to go ahead and update the projects to the newer NET Framework version (4.8). If you intend to upgrade the projects then you might as well go this route anyway. It'll just update the project files.
Now the fun starts. In general if you previously targeted NET 4.x then updating to the newer framework is pain free and won't require any changes beyond the target framework update (which is automatic or can be done via the project properties UI) and updating any NuGet dependencies to the newest version (which you should do anyway). Of course any code that was deprecated or removed will no longer work and there may be language changes that cause some code to fail compilation but these are generally pretty easy to fix up. Once you're done though the upgrades are complete.
For the pre-NET 4 projects though you may have more work to do. Ideally it'll just auto-upgrade and you follow the previous steps. However if you have config files and they had settings that are no longer supported then you need to clean them up. Most likely there are also project settings that aren't needed anymore but shouldn't hurt anything. Honestly it might be easier to just create a new NET 4.x project and copy all the code over and then remove the old project. This will at least clean out any legacy settings.
What people are referring to when they say the project file has changed is when you migrate from NET Framework 4.x to NET Core/5/6/7/8. The newer frameworks use an SDK-format which is quite a bit different and simpler. Once you are ready to upgrade to the newest NET Core runtime then you'll have to convert. Fortunately MS has an Upgrade Assistant extension you can install that can make the project (and some related) changes for you. Alternatively there are a couple of custom extensions floating around that do it as well. Ultimately it is a big change so I wouldn't bother until you are on NET Framework 4.x.