Purchase new SSL Certificate before existing one expires

Solution 1:

You can just request a new certificate, and run both certificates at the same time.
In fact this is quite common for applications that need to be allowed to run without downtime, that require new certificates.

If you install both on one server, the old certificate will be ignored, in favor of the new one.
Datasprings has a nice write-up about certificate renewal.

Solution 2:

Some CAs, I know DigiCert is one, will carry over the remaining time on your existing cert to the new one.

If you're using such a CA, then just go ahead and request the new certificate.

If you're not, then the only issue that would arise is that you paid twice for the left over time. There are no technical issues to take into account.