Bob Dylan's always said the times are a-changin' -- and apparently his will has as well ... 'cause an old copy of the legal document has just hit the auction block.

Moments in Time -- a collector's auction house -- has a copy of Dylan's will from 1975 for sale ... signed by Bob as "Robert Dylan," using his full legal name because it's a formal document.

Check it out ... Dylan leaves his brother, David Zimmerman, $50K and the house he lives in -- and, he makes provisions for any future children he may have well. It appears he had five when this will was drafted.

It's unclear why Bob decided to change his will ... but, clearly he's moved on from this version -- hence why it's on the auction block.

The asking price is $27,500 ... so a rolling stone is going to need quite a chunk of change to pick up this rock and roll memorabilia.

Obviously, Dylan is on a lot of people's minds after the recent release of "A Complete Unknown" -- the Bob Dylan biopic starring Hollywood hearttrob Timothée Chalamet.

Bob wasn't knock, knock, knockin' on Heaven's door in 1975 ... but, now a fan can purchase his post-mortem plan from way back in the day!