Not this old chestnut again.
This idea applies to older type saws that run below 25:1 mix ratios but had little relevance to modern saws that use 25:1 or higher ratios.
Changes in air pressure due to changes in weather are around 4%. This means that weather changes can vary the gas/air ratios by 4% and most saws are designed to accommodate this. On top of this the gas/air ratio that saws get is significantly altered by how clean the air filter is. Most saws will keep running (maybe not well but they will keep running) even with very dirty air filters further demonstrating the gas/air ratio (heading in the richer direction) does not make as significant a difference as one thinks.
Here's a table that shows the "Mix ratio", and then the "% of gas in the mix" in the middle column, and then the "change in the % of gas in the mix" from a 50:1 ratio.
Many people have a limited idea what lube/gas ratios mean. Then think if they change the mix ratio from 40:1 to 20:1 that they have substantially reduced the amount of gas in the mix where as it changes from 97.6 gas to 95.2% gas so the change is only 2.4% ie hardly anything and most saws will not notice this.
To change the gas/air ratio by more than 4%, for a given atmospheric pressure, the mix ratio needs to be changed over a range between 50:1 to 16:1 - see yellow colored entry in table.
Of course a saw is unlikely to accommodate i) changes wide in air pressure, ii) wide changes in mix ratio, and ii) changes in air filter cleanliness, but most should be able to cope with changes from 50:1 to 30:1 without any problems and certainly without changing jets.
If there are any issues it comes from going the other way.
If a saw is tuned with dirty filter, low mix ratios, and at low atmospheric pressure and the same saw;
- has it's air filter cleaned
- is fueled with a substantially higher mix ratio
- in higher atmospheric pressure clean
Then that is asking for trouble.