Minecraft – Lost in Creative mode

Not only is this unfortunately impossible with your current setup, it's impossible to reach a high overworld portal from the Nether as of 1.2.4:

As of the 1.2 update, there is a confirmed bug in which the "portal search" performed during travel back from the nether will not find any overworld portal above a height of 128. Thus, it is not possible to travel back to the overworld and land at a portal which is above y = 128.

From the Minecraft Wiki article.

Additionally, without mods, there is currently no way to move upwards without some sort of connecting structure, save perhaps a vertical TNT cannon, and it seems to me that you want something a bit more usable than that.

Now, in theory, once this bug is fixed, you could build a transport bridge a few blocks out in the Nether and the same number of block multiplied by 8 in the overworld coordinated just so that the higher platform is closer to the new portal, but it would be complex to do.

Until then, you're stuck with regular transportation.

The distance that your new portal needs to be away from the old one isn't terribly large, only 128 blocks, but it's measured in terms of the destination world; every block in the Nether is worth eight in the overworld, meaning you'll need to move around 1024 blocks or so away to keep a new portal from joining the one that already exists in the Nether.

I say "around 1024 blocks or so", because the existing portal on the nether side may already be some distance away from the location that would best match up with the existing Overworld portal.

There are, however, a few other options. You could try building a bridge across the lava on the Nether side. Painful and awkward, but once across, you could build a new Nether portal on that end, destroy the Nether-side one that exists (just punching it to turn it off should suffice), and hope that the new Nether-side portal successfully connects to your existing one. Alternatively, you might be able to make a suicide run through the portal, destroy the island so it's no longer a valid spot (and also destroy the portal on that side, too), then once you die and respawn on the Overworld side, a quick trip through should trigger the creation of a whole new Nether-side portal, which might not be in such a poor location.