If your roof is approach end of life, you have two options: replacement or overlay.
A replacement is when the roofers remove the existing waterproofing system (shingles and underlayment) down to the wood decking of the roof. Then they resheet or prep existing sheathing, and re-roof with new underlayment and shingles (as well as other accessories).
Generally, this is more costly, but the solution lasts longer. Most new roofs will need a replacement.
But sometimes a roof can have the perfect conditions that make an overlay viable. The tear-off would be skipped. But you need a certain set of conditions for your roof to qualify for an overlay.
The roof deck needs to be solid
Many homes were built many decades ago before plywood. These roofs usually have shiplap or skip-sheathing. In each case, an overlay is not viable since the nails for the new shingles won’t reliably secure to those substrates.
If your roof is not plywood substrate, you’ll need a total roof replacement that includes resheeting with plywood (or OSB).
The attic must be properly ventilated
The last thing you want to do is install new roofing over an attic that doesn’t vent well. This is because an improperly ventilated attic will cause condensation buildup on the wood roof deck, turning to rot. Some rot can get so bad that a roofer walking on the roof could potentially fall through.
Since air must escape somehow, often the roof will look like it’s being pushed outward if the attic isn’t ventilated well. If you’ve ever noticed shingles curling up, there’s your sign.
No quality of roofing material can overcome the problems associated with poor ventilation. It makes sense that all roofing manufacturers void (or significantly decrease) warranty coverage with problems are related to venting.
Even though a lot of old homes don’t have the kind of eave ventilation that’s common today, the issue can be fixed with roof-side intake ventilation during a roof replacement.
The plywood must be intact
If existing plywood roof deck is showing any signs of not performing perfectly, an overlay is likely to not last that long because the problems are likely coming from the inside. Not addressing the underlying cause of worsening roof deck will just mean that problem festers.
In general terms, your roof can be a candidate for an overlay if it’s been performing without any issues for a decade or so, and it’s just the shingles that are losing their ability to weatherproof in the near future. Then you could install new shingles over the top and get another decade or so life out of it.