OK. Rules as I understand them:
Weapon fusions applied directly to a weapon can be migrated to a higher tier weapon. The cost of doing this is a bit wonky, but that isn't the point of this question.
A weapon fusion seal is an item with a weapon fusion on it. The item can then be attached to a weapon. The tier of the seal item determines the price of the weapon fusion as well as the max tier of the weapon that it can be applied to. The cost for the complete fusion seal is 110% of the cost for just the weapon fusion.
So, if I have a lower tier weapon fusion seal, can I craft a new fusion seal item and transfer the weapon fusion to it at the reduced cost for transferring weapon fusions? The cost of the fusion seal item would be the 10% of the cost of the weapon fusion for that tier (which is consistent for the 110% cost for both the fusion seal item and the weapon fusion together).
So: houserule or RAW?
Honestly, I wish they would just fix the prices, and make the weapon fusion work more like Armor upgrades. Most of the weapon fusion effects are more of a utility variety in the first place. Few of them are direct damage boosts.
As others mention, the whole rules around weapon fusions make them pretty complicated. I think making it one flat price to purchase the fusion with installation fee (or free if the player has the skill) with minimum item level potentially being reevaluated is a better way to go. And you still end up being limited by having a number of fusions levels equal to the items level.
Honestly, the rules are pretty weird. Like you should always hang on to a weapon of the lowest level that is required to put a fusion onto, because buying the fusion for that lower item level weapon and then transferring it onto your main weapon is substantially cheaper than purchasing for your main weapon. Usually even if it means that you have to outright buy the lower level weapon to do the trick.
I really wish fusions and fusion seals were wrapped up together in one.
You can have a combined fusion level = item level in any one weapon, and you should be able to swap them between weapons once a day at no cost.
That way you can have a collection of useful fusions you grab at the start of an adventuring day, or you can just have your favorites installed in whatever weapon you've picked up and are using - ignoring and selling the rest.