The truth is that we really never know. The blaster may be meant to be the exact same in all movies and they never expected fans to get so wrapped up in the details. Remember that Solo's bunker blaster is very different in those scenes from his hero blaster in ROTJ, not to mention smaller variations in stunt props and the same in the other movies including the Greedo Killer differences. At know time in ROTJ do we assume he switch blaster before and after he left the bunker.
An argument can be made that Han would require a new blaster after he left his old one on Cloud City. But one could argue that it was later retrieved some way the same as it appears Luke's X-Wing Helmet and ship somehow made it back too. Even if you believe that the one blaster in ESB was left behind and never retrieved, the idea might be that his replacement was to be perceived as the exact change. In short, what the prop people wanted you to think may be very different from what is actually observed.
It would be really cool if they made a bunch of short stories in the new canon explaining how and why all the props change through the movies lol. It does get complicated when you think about the greedo killer and the bunker blaster though.