It must be 25 years or so at least since the last reformation took place... certainly 2617+2614 and 2610+2613 have been 'married' since fairly early on in the careers in 1994 (they worked an IRRS special as such in June of that year).
I think 2605+2616 and 2615+2606 were later swaps, either late 90s or early 2000s?
A few 2700s got mixed around too, at least one such change occurred after 2716 infamously tried to jump off the end of the carriage sidings in Connolly.
2800s have remained 'properly matched' but I think in the very, very early days there might have been some sets 'mixed up' during delivery.
Not directly related but Tokyu nonetheless, there are two 8620 DART sets running 'mixed' at the moment, 8621+8521+8526+8626 and 8625+8525+8522+8622, I believe related to a signalling equipment trial but someone will be able to clarify the specifics. There were two misformed 29000 sets for a while for the same reason but these have since been 'corrected'.