2.6 changed the way relationships worked, so the 2 sites on 2.5.2 will need some work before upgrade is possible.
When I started using EE 2 Core the license allowed for the use on non-profit sites as long as you displayed “Powered by ExpressionEngine”.
But there were certain addons that were essential for what I needed to do and to improve the user experience, like Matrix, WYGWAM, Assets, Geotagger, CE-Image, CE-Tweet and Date Field Filter Plus a few free addons like TitleMaster, Low Seg2Cat, MX cloner, Switchee and more.
I do also have a number of these club sites on fully paid licences and I even paid to upgrade some of them to EE3, but the long delays in porting 3rd party addons and problems with EE3 for a long time meant I lost the will to go down the upgrade path.
The new license terms mean I can’t use Core 3, so it will cost $300, with current exchange rates that’s £240, plus the cost of new addon licences.
I was looking for the easiest route to ensure that these sites weren’t at risk from any security issues, but it looks like the easiest/cheapest option will be to rebuild them from scratch with Craft.