I had been operating under the assumption that tech upgrades were just a renaming of modifications, but on page 20 of the new rulebook, that appears to not be the case. So can a ship that has the tech icon use a tech upgrade and a mod? And any ship that came out prior to now can't use tech?

Correct.

Tech is a new type of upgrade. A pilot's card must have this icon to use these upgrades. One modification may be placed on ANY ship (inside the limitations that may be expressed on the modification card). These are two different types of upgrades and both may be used on the same ship when legal.

Cool. I thought I had seen people earlier talking about using Weapons Guidance on an old ship and got all confused.

Now, there is talk of the 'missing' SKUs, and speculation, as with the original Wave 1 launch, that those might be individual T-70 and TIE/fo and new pilots and upgrades. If that is the case, which seems logical, I would NOT be shocked to see an astromech or something that allowed you to add a [Tech] icon to your upgrade bar. But that is purely speculation.

Cool. I thought I had seen people earlier talking about using Weapons Guidance on an old ship and got all confused.

Currently, only the new TIE/fo and the T-70 X-wing have pilots with the appropriate icon to use Weapons Guidance.