Relevant things for note checkers, such as the note's dimensions, watermark and magnetic strip etc. would stay the same, wouldn't they?
What exactly do your note acceptors check?
no, in canada, it's quite the opposite. Bill scanning is done completely optically using ultraviolet, infrared and visible light frequencies (typically red and blue). only some of the cheapest acceptors only checked for a conductive/magnetic ink ID code printed on the backside... and that feature was phased out back in 2006 when the first polymer bills came into circulation. all the rest of the security features are for human verification of a bill.
currently, when a bill is inserted into an acceptor, a set of LED lights turn on and begin shining light on the bill. some of this is passed through the bill...some reflected off.
a drive motor with rotational feedback starts feeding the bill in. over the length of the bill, it's emissive and transmissive properties of all the light frequencies shone upon it are recorded with sensors every motor pulse... about once every 2-3 mm of length bill feed across its width for the entire length (this measures the length of the bill because a bill should only block the sensor for a certain number of pulses.) essentially ending up with 100's of numbers ranging from 0 to 255 (a sensor that sees absolutely zero light to a sensor completely open to light) and the dataset is compared to a "standard" master set stored in memory.
depending on the acceptance rate setting (how strict or loose it is), how far these values can deviate from the master stored in memory, garners whether the bill is accepted as legit or not. currently, a counterfeit bill would have to be so close to the original in makeup (materials,inks, etc) that producing it good enough to be fooled by a scanner of this type, would be so expensive it would cost more than the bill itself.
bill folds and other damage is accounted for in the datasets, (this set of numbers in this middle area can have more deviation because this is the middle of the bill and often folded here kinda thing) so that wrinkled and used notes still accept. it's actually quite often that a pristine bill does not accept because its "too good" to the dataset and scrunching it up a bit will actually make it accept.
if the bill it has scanned is deemed acceptable, it issues the command to -whatever- that credit is to be issued and the bill is fed further into the acceptor to be stored...otherwise the feed motor is reversed and it's returned.
therefore, keeping the bill identical and only changing the portrait on it, still changes the emissive/reflective properties of the bill for the area of the portrait, rendering the sampled scan invalid against the master dataset stored inside.
I worked with ICT back in 2015 to create master datasets to create the first updates when canada switched to polymer bills because i live in one of the biggest cities, we got publically issued poly bills in main circulation before everyone else did. their initial firmware releases had very VERY poor acceptance due to the fact that they had used a commemorative collector bill issued by the bank of canada that was not intended for use as circulated currency, so deviated from the actual bills that were released and used for circulation.
but yeh, having to reprogram and recalibrate 300 machines acceptors, is going to take a hot minute and a pile of money since some are EOL and not going to get updates so will have to be replaced. we are currently looking at a digital Credit, debit, RFID card reader solution for those machines because the cost of BA replacement is about the same and offers more choice despite not taking physical currency.
as for the coins, as long as they don't change the metallurgical content and the weight doesn't change, we won't have to worry too much about reprogramming those. time will tell. worst case, i have to "teach" program them (where you insert 8-10 coins of the type you want to accept) it just means a laptop lives permanently in the truck for a while.
so yeh, this is a legit concern for us.