AFAIK:

Masterworking does not affect infusion.

Most of the time, an older seasonal item with the same name as a newer seasonal item (i.e., the only difference is the seasonal mod slot) will require an upgrade module.

Items coming directly from the season pass track, either infusing or being infused, seem to always require an upgrade module.

OK. With weapons, if you have the same weapon, you can infuse it for 1000 glimmer.

I thought this was also the case for armor, but only seems to work some of the time. Does anyone know what the actual current rules are?

Here are my few known data points:

* seven seraphs non-MW'd armor, infused up by a seven seraphs non-MW'd armor = 1000 glimmer
* Ancient Apocalypse Mark 2.0 MW'd , infused up by Ancient Apocalypse non-MW'd = 1000 glimmer
* Substitution alloy greaves MW'd, infused up by substitution alloy greaves non-MW'd = 1 upgrade module
* armor 1.0 4/5 MW, infused up by current season armor 2.0 version non-MW'd = 1 upgrade module

Does it have to do with Masterworked gear? Does it have to do with being the same elemental affinity??? I can't figure it out.

I looked around online and I couldn't find any current season information on upgrading.

I was hoping to save on upgrade modules by just spending glimmer, but I had only very few opportunities where that was possible. The rules seem confusing and kind of inconsistent to me so far.

The exact same thing happened to me, so I know what's going on. Unfortunately, I've got bad news. Any armor that is Year 1/1.0 requires an Upgrade Module for infusion, even if it's the exact same piece. So if you're trying to infuse a Y2 into a Y1, it'll still cost you an Upgrade Module. They have to be the same year for the 1K glimmer price. It sucks, but at least we can still use them.