There are two distinct questions here:
- Could you upgrade to 256G or 500GB
- Should you upgrade to 256GB or 500GB
Let's tackle this backwards and start with the question that precariously walks the line of being an opinion and out of scope....
I'm going to take the official stance and say no.
BTW neither Apple or Salers told me whether it is possible or not
I don't know who "Salers" is but there's a reason they're not replying: Apple's official position on the matter is that it was upgradable at the time of purchase. The MacBook Air 11" Early 2015 uses a proprietary PCIe 2.0 x2 interface meaning that from Apple's and any authorized service centers the only SSD that is supposed to go in there is an Apple PCIe 2.0 SSD module. Since there were no after purchase options available, they're not going to reply to you.
on Apple's web site it says it can be upgraded to a maximum to 256GB.
So, despite what's stated by Apple, you're looking for advice to see if you can circumvent this specification.
Technically, you can install another Apple SSD module if it's PCIe 2.0 x2 provided it physically matches.
Now, looking at this from an unofficial perspective....
I want to upgrade it to 500GB with a third party SSD (970 EVO Plus) but, ... Should I upgrade it to 500GB or stick with a 256GB upgrade?
Neither of these upgrades, as described, are compatible because they don't conform to the Apple proprietary interface. They are off the shelf PCIe M.2 NVME SSDs So, to connect any of these drives to the proprietary interface you'll need an NVME adapter. This will convert the industry standard interface to the Apple interface. However...
You can't just grab this converter, plug it in and blast away with your new found storage and latest operating system. You have to install (usually incrementally) macOS upgrades onto the original SSD before you can migrate to the new SSD. Why? Firmware and driver updates need to be applied to ensure your Mac can "see" this storage. If you don't do this in the right order, your Mac won't simply won't be able to access the storage for you to use.
This is so finicky, that they continue evaluating compatibility with each new OS roll out.
The guy on the Internet said....
The question here is "do you have the technical skill to do this?" Like Apple and the reseller/service provider you took it to, they are staying mum on the issue as will I because like them, I don't want to be "the guy on the Internet who said this can be done" then you attempt it and end up with a Space Grey paper weight.
This is not a knock on you whatsoever, but you must take stock in your technical ability to do this. If Apple or authorized sellers won't come near this topic , I'm inclined to follow their lead in this case. If you absolutely must have this done, you need to seek out a trusted technical resource that can handle this for you.
If you do this, and decide to step out of Apple's "box," remember it will be at your own risk and as always, YMMV. Do your research before purchasing parts or disassembling things.
And...advice I always give people when asking what they should buy when purchasing a piece of equipment:
Buy for what you think you'll need 3 years from now, not for what you need today.