Connecting to PXC from legacy application and server


as usually happens, customers find out they need to upgrade their systems right before the deadline.
In our case, we have been contacted by a small firm. They need their legacy e-commerce made ready for EU GDPR privacy law, taking effect on May 25.
There won’t be time enough to upgrade their whole server (large codebase, lots of manually compiled stuff) but we want to:
[]Install Ubuntu 18.04 LTS servers
]Install latest Percona PXC on them
[]Configure Percona PXC for data at rest encryption + key vault server etc. etc. We have done other configurations like this.
]Reconfigure the legacy software to point to the new servers cluster IP.
But we have a doubt: shall their legacy server MySQL client(s) connect and work, considering the old server is Ubuntu 12.04 with (Nginx + ) PHP 5.3?

If someone of you has got similar experience, we’d be glad to hear it. We’d love to avoid any unneded e-commerce downtime, especially if the downtime did no good because we find out their legacy MySQL client cannot work with latest Percona.
As I said, upgrading their server is out of the equation for now.

Thanks in advance.