Upgrading a Traditional Magic Item in a Non-Traditional Slot

[JDDyslexia]

I figured I would ask because I'm not sure what rule guidelines this would fall under.

So, from a module bonus chronicle sheet, I got access to a very handy item for the particular character that has it. Here's the exact wording:

necklace of resistance +2 (6,000 gp; as a cloak of resistance +2, except it occupies the neck slot)

My current issue is that I'd like to have a better bonus to my saves. Now, I see that this price is 50% higher than the standard cloak of resistance +2. Can I use this as a guideline to improve the bonus on this necklace as the following?

+3 => Item Cost as 13,500gp, thus 7,500gp to upgrade
+4 => Item Cost as 24,000gp, thus an additional 10,500gp to upgrade
+5 => Item Cost as 37,500gp, thus an additional 13,500gp to upgrade

If it matters, the item listing on the chronicle sheet doesn't have a custom item stat block, nor is it part of a special boon. It's typed as I typed it above in the list of equipment available for purchase on the chronicle sheet at the bottom

[BigNorseWolf]

I don't believe you can upgrade these, as the +2 is just as baked into the item as anything else.

[Belafon]

I have to lean towards “no.”

There have been several “non-traditional slot” items on chronicles that specifically include upgrading instructions. Pretty sure I remember a necklace of vast intelligence +4. The chronicle specified that it could be upgraded to +6 and included the cost to do so.