Ewan ko lang. Maybe they will reuse the other series no longer in use. I think it needs a congressional bill to amend the current plate format for motor vehicles. Di lang ako sure.
After they exhaust all unused letters (I,O, U, etc.) in the "N" series, they would also do it on "P", "T", "W", and back to "Z". I give it five or more years to go before they exhaust their current system. By then, if they can't change the format, they would decide to use more combination of other letters.
I did some computations, I'm not sure though if it's correct.
Here's what i found out; Our current license plate format of "AAA-000" (3 letters + 3 numbers) has a total of 17,576,000 combinations, even if this format was used since the 1900's I doubt the Philippines ever had that much vehicles. So, if the LTO is dead set on exhausting all possible combinations, it would take quite a looonngg time before the combinations run out, considering only 200 thousand plus vehicles enter the road each year (that includes local car sales, subic imports and backyard assembled jeepneys and auv's)