Might be more of a rules question.
I built a Dueling Sword, Tactical. Level 2, 1d6, and I made it out of a special material, Noqual. So +1 damage vs magical constructs and undead. Not a great sword, but I named him due to the circumstances we were in.
Now Starfinder is not like Pathfinder. I can't just slap a +1 on it every few levels and be good. At level 8 I've gotta upgrade to a Dueling Sword, Buzzblade so I can put a fusion on it.
To my understanding of the rules, this means the buzzblade is 9500 credits, so I need 9500 UPBs. The current sword can be deconstructed for 10% of it's value, so I could cut 47 UPBs off, not including the Noqual aspect which would be 50 more UPBs, however I would need 500 more UPBs to keep the special material (or another hunk of it, but I don't wanna drain our party supply of it). Then adding the Interposing weapon fusion woukd be 2300 UPBs, and the aide of one of the team's Mysticism characters.
However, that's deconstructing the sword entirely, which is destroying it and it's story to start anew.
Then come level 11 gotta upgrade to a Dueling Sword, Ultrathin. So 26000 + 500 (Noqual) credits or UPBs, deconstructing the old one for 950 + 50 this time. Then there's the Interposing fusion which can be transferred? Can you transfer a fusion from an item you're taking apart to put back onto it when you're done? Either way, transfering it from old to new gets a 50% discount so it's (4880/2)=2440. So 2440 that needs to be credits? Or would it be constructed in UPBs?
At this point my head is spinning a bit. I'm not sure I'm doing it right, or if there's a better option. Plus I think I wanna also add the Flaming fusion for 4880 so it has two damage types to help bypass DR.
Level 15 you do it all over again with a 109250 credit sword and 17800 credit fusions (with 500 for the material). Plus the question on whether you can transfer 2 fusions at once. Then (if we get to this high a level) one final time at level 18 for 331200 and 60300 respectively (again with 500 material).
Am I on the right page? Or is there a better way to keep the same sword along my journey?
What you describe is about right, although I didn't check the exact numbers, but the mechanical process sounds right. Although depending on your GM, you could probably simply refluff "scavenging" as "upgrading". In any case, as long as you spend the same amount of credits and UPB in the end, it doesn't matter all that much how the process is described.
As for transferring the fusion from a weapon to an upgraded version of itself, I'll note from the CRB:
Characters trained in Engineering or
Mysticism can also install fusions, if necessary (for instance, if
the PCs find an unused fusion as part of a treasure cache, or in
the case of a character who used Mysticism to craft a fusion;
see page 235 for more about crafting items). In either case,
installing or transferring a fusion takes about 10 minutes of
uninterrupted tinkering.
Fusions can exist independent of the weapon, otherwise it'd be impossible to find an unused one as part of a treasure cache. So when you transfer a fusion, at some point you must be holding the thing in your hand between weapons. I don't see why you can't store it, upgrade the weapon, and then pay the transfer fee to install it in the better version of the weapon.
As for paying for the fusion transfer, if its a store doing it, you pay in credits. If you have someone in the party doing it, its paid in UPB for the material components in the ritual (the UPB become the reagents for the magic). I'm pretty sure magic doesn't accept credit. :)
The Paizo Roleplaying Rule: Get the mechanics you want, and then add fluff to it.
Mechanically, you want a good weapon. Get it.
Fluff-wise, you want to not throw away your weapon. Just say it is an upgrade like in Pathfinder. Or if your GM is going to be a knob about it, hang in on the wall in your ship's quarters.
Keep melting it down into its component 10% UPBs and reforging it?
The others beat me to it, but I'm pretty much of the same mind: just follow the rules and flavor it as an upgrade.