Why do you want to migrate in the first place? You talk about the 'old version' and the 'new version', but that is not correct and suggests you may be confused about the two versions. Lightroom (cloud) is not the new version and Lightroom Classic the old one, both are current, but different applications. You can migrate if you want to, but you don't have to and in your case you probably should not.
Before committing to Lightroom Mac / Win for desktop you should first check how much space your existing photos take up on hard disk(s). If more than 1TB then you'll need to factor in the additional cost above the 1TB subscription. You should also check whether your current internet connection is sufficient to cope with number of images you'll be syncing to the Adobe cloud. In particular, the upload speed available to you. Typically, at least in UK and Europe the upload speed may be as little as 1/10 of the download speed. Also, with 100,000 images you can expect the process to take days rather than hours.
Lightroom Mac / Win will honor any existing develop adjustments you've already applied. However, your existing develop prsets will need to be upgraded to the format now used by Lightroom Mac / Win, Lightroom Classic and Camera Raw. I can't recall if this happens automatically duing migration or if you need to do it afterwards, but it's relatively easy if latter applies.
If your keyword (Tags) are hierarchicial they'll automatically be converted to the flat format that Lightroom Mac / Win supports.
Thanks for the quick reply. I realise I was a bit unclear. I currently have all of my folders in LR 5 and want to migrate to LRCC.
There is no LRCC anymore. There is Lightroom Classic, which is the latest version of the Lightroom you now use. And there is Lightroom (cloud) which is an entirely different application. To upgrade to Lightroom Classic is easy. Get the Photography plan, install Lightroom Classic, start it and open you catalog. Lightroom Classic will tell you it needs to upgrade it. Let is upgrade it and you're done.
Migration to Lightroom (cloud) is a bit more complicated. You cannot migrate a Lightroom 5 catalog, so you would still have to install (a trial version of) Lightroom Classic to upgrade your catalog. Then you can migrate it to Lightroom.