>start a game with magical gear, decide that +1 sword is a family heirloom
>fight trolls with it, fight minotaurs with it, even fight a dragon with it
>travel through ice caves, volcanoes, even sailed across an underground sea with it
>wear it at your hip during the paladin's wedding, wear it during the wizard's funeral
>keep it by you while you sleep, always carry it in your dreams
>dove after it when the dragon turtle capsized your ship, swearing that you'd never let it go
>find a +2 sword
this is you. You are a digusting human being. Enjoy your stay.
Upgrade ancestral sword or start a legend by passing it down to a newbie adventurer who passes it to a newbie adventurer. Alternatively keep it on your back and the +2 at your hip.
Selling the 8,000 gp +2 sword only nets you 4,000 gp, and a wizard wouldn't upgrade a sword for the cost of creating it since it costs him XP. At best, deducting the 2,000gp from the base +1 weapon from 8,000gp leaves you with a 6,000gp gap, which with the 4,000gp from selling the +2 sword still leaves you with needing to make up a 2,000gp difference.
A creator can add new magical abilities to a magic item with no restrictions. The cost to do this is the same as if the item was not magical. Thus, a +1 longsword can be made into a +2 vorpal longsword, with the cost to create it being equal to that of a +2 vorpal sword minus the cost of a +1 sword."
You can turn a +1 sword into a +2 sword for the difference between the cost of making a +2 sword (4,000) and a +1 sword (1,000). In this case that means it costs 3,000 gold pieces, meaning you can sell the +2 sword and use the money to upgrade your existing sword, with a bit of change leftover.
As a GM I've always been pretty lenient about letting my players transfer enhancements as long as they remain on good terms with the local item-wizard and as long as their WBL remains balanced.