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I have the opportunity to upgrade from VDSL to any of 1/10/25 GbE fiber. I have only an SFP router presently (Edgerouter 4), but would like to have the service provider upgrade to optimally the 25 service, and replace the router on my side later.
Can you run SFP28 or SFP+ modules in an SFP cage at the lower speeds? Alternatively, can you find SFP modules that sync against a faster physical interface?
Generally, slots may be downward compatible but are not upward compatible. Of course, there are exceptions.
Since a module faster than the slot doesn't really provide any benefit, you should use a module that matches the best mutual mode supported by the far side - your ISP should be able to tell you which these are (possibly single-strand 1000BASE-BX or duplex 1000BASE-LX).
In no way can you link a 1G SFP port with a 10G-only counterpart, regardless of what module you try to fit.
As mentioned, the link provided by the ISP will be configured for 1 Gigabit, 10 Gigabit, or 25 Gigabit. Your equipment much match the link speed they provision for you. If your equipment doesn't match the link speed and type of media (single mode fiber most likely), then it will simply not bring up the link.
Additionally, purchasing a service your equipment can't use will only result in wasted money and a prolonged outage of your internet service. You will have to upgrade your equipment to match the service provided by the ISP or have the ISP revert the service back to your old account that matches your equipment.
As a recommendation, it is extremely rare to actually be able to use more than a few hundred megabit on a personal internet service for home or small office use. I work for an ISP and find that most users that buy 1 Gigabit service tend to use at most 40-150 Megabit on average and have very short peaks in the 500 megabit range, but only for a few minutes at a time. If you find your current service is routinely not sufficient for your needs and you often need more than 400-700 megabit of service, then an upgrade to 1 Gigabit or more might be worth the cost. Otherwise it is probably not worth the additional cost. If this service is for an office with multiple users, make sure the usage is actually work related usage and not simply people downloading non-work related materials all day.
