I think older devices will keep the old Siri. When I bought my daily driver 16e, I learned that the reason it has the A18 processor (even though it is not considered a flagship device) is specifically so it would be able to handle Apple Intelligence. That tells me that the older processors (maybe pre-18?) are not able to use Apple Intelligence - they're stuck with what they already have.
Note that I'm in no way an expert and may be off the wall, but deductive reasoning to me says the old Siri will remain on the 15 and older.