Hi,
I think you are mixing up two things.
The first is the PON. This is simply a bit of fibre which goes from an exchange of some sort to a number of properties. This is the time consuming and expensive bit to install as it involves physically installing fibre cabling all over the area you want to serve. Modifying this is hard and expensive so you want to install it and not touch it again.
The second is the technology you install at the end points of the PON - this is the cheap(er) and easy bit. There are a whole host of different technologies, GPON, EPON, XG-PON, XGS-PON, 25G PON, 50G PON, etc. etc. Many of those technologies use different frequencies (think colours) of light so they can be used at the same time. It is relatively easy to upgrade this as you only need to touch the exchange and the customers' equipment, not the expensive to install and modify fibre network.
Visible light ranges from a wavelength of around 380nm (for violet) to 700nm for red light. PONs generally use light over 1000nm which many people would call infra-red. GPON uses 1480-1500nm in the downstream direction from the exchange and 1290-1330nm in the upstream direction. XGS-PON uses 1577nm downstream and 1270nm upstream. 25G-PON uses 1358nm downstream with the option of either 1300, 1270 or 1286nm upstream.
Because those three technologies use different colours of light (providing the third 25G PON upstream option is chosen) they can share the same fibre at the same time and aren't even aware of each other. To do this, all you need is the right splitter at the exchange to split the different colours of light to the different head end types (note that these may actually be in the same piece of hardware so the splitter might be internal) and you then install whatever piece of technology is appropriate at the customer end for the service you want to provide, GPON, XGS-PON or 25G-PON, etc.
You only need to worry about phasing out old PON technologies if you either need the frequency for something else (e.g. if someone invented 500G PON which used the same colour light as GPON) or it is adding too much cost (e.g. if the equipment at the exchange that does GPON, XPS-PON and 25G PON becomes twice the price of the equipment that only does XGS-PON and 25G PON because GPON has become obsolete).
It should be noted that because installing the fibre is the difficult bit, in general a PON network's value is related to how many properties it serves rather than the technology installed.