What are the operational limitations on ADS-B DO-260A (as compared to DO-260B)?
If the aircraft is subject to the ADS-B mandate of Europe, it will not be allowed to operate in European airspace after June 7, 2020 if it does not have a certified EUROCAE ED-102A (equivalent to RTCA DO-260B) compliant ADS-B transmitter. Aircraft with a maximum take-off weight exceeding 5700 kg or maximum operating true airspeed exceeding 250 knots are subject to the mandate. 

The USA rule is a bit different, as it allows ADS-B on universal access transceivers (UAT) below 18 000 ft which is different from the 1090ES technology used in DO-260. You can do without ADS-B in the USA after January 1, 2020, if you never fly higher than 10 000ft MSL (exceeding 10 000ft MSL is allowed as long as you don't exceed 2 500ft AGL), you do not enter or overfly class C airspace and keep further than 30 NM from class B airspace. 
If your aircraft is 1090ES ADS-B equipped it must be DO-260B compliant; if it UAT ADS-B equipped it must be DO-282B compliant. 

The difference between DO-260A and DO-260B are many small changes. This list is not exhaustive:


change in the way position integrity is encoded, 
addition of barometric pressure setting,
new encoding aircraft length / width
change in encoding of speed on surface 


Typically the update from DO-260A to DO-260B requires a firmware update and some configuration changes. However, note that the mandates include more requirements than just the ADS-B protocol version. There are requirements on the integrity of your position source (GPS) as well, and the maximum latency in the position transmission chain. In the worst case you are up for a rewiring of the GPS or even installing a new GPS.