The main obstacle is php versions. For v8 you can take advantage of php7+, typically php7.2.
For legacy c5.6 sites, you are stuck with <=php5.6, unless you update the sites to c5.6.4.0.

c5.6.4.0 is a pretty safe update for legacy sites as long as you go through 5.6.3.5 first. You need to run this update before switching php to php7.2. The main obstacles are likely to be some addons or custom application code that are not compatible with php7.2, so be prepared to make some small (or larger) code changes depending on what happens.

Making a development clone to test the update is the safest route, depending on how critical site continuity is.

Hello Again (John),

I have encountered a situation where the software will not update past 5.6.3.5 on several applications...states ' Your site is already up to date'.

Any thoughts or 'workarounds' on this?
Thanks again.

5.6.3.5 is the last automated update for legacy sites. For 5.6.4.0, you need to download the zip and do a manual update.

Thanks again. That explains it. Cheers for taking the time.

Thank you for your response. That's very helpful and much appreciated.