what are you making? that will make a difference according my book.

I also signed up and passed their canning course, it's a basic course and free. I still learned a few things that have changed since I first learned to can. And other things that I never tried. I've only been pressuring canning for less than 10 years, water-bathed before that.

I learned with BWB, back when you could do everything in it, but also lots of people got sick from doing things unsafely.

That said, with highly acidic foods, such as most jams and jellies, some choose to ignore that guideline because the primary safety is provided by the acid level pH not the processing time. There are exceptions as there are some low acid fruits and of course it is never done when working with low-acid vegetables and such.

So technically no increasing the jar size. Instead you are supposed to buy the jars called for or use a recipe that provides a pint jar processing time. But in actual practice it is done but at your own risk since molds are the primary safety issue.

In this particular case only (given your altitude) I would start off with sterilized pint jars (not normally required) to provide an extra safety margin and process for the time called for in the recipe. No additional adjustment for altitude is required. 15 min, is sufficient since both cherry and plum jams call for only 10 mins for pints at your altitude.

Dave