does raising slow cooker pot roast from low to high halfway thru keep meat tender or will it turn into a boiled lump?

Slow cookers do not generally have exact temperatures that relate to "low" or "high" settings. Are you trying to speed the process? You can play with the temperature a bit, but when you are talking about a tough cut of meat, especially a larger chunk, high heat will turn it into a tough mess. If you want to put it into the oven, you could probably go as high as 275, maybe 300 to speed things up (though I often cook shoulder and other tough cuts closer to 225), but I know my crockpot has turned shoulder into a tough lump more than once, and once this happens there is no real fix. (Well, you can slice it into small shreds and douse it in sauce, but that is not the same as melt in your mouth, dissolved collagen, thick rich post roast.) My advice is to go long and slow, it may not be ready when you are but it will be worth the wait.

I think that even the high temp on a slow cooker is relatively low, so you would not have the result you are afraid of.

My guess is actually the opposite... Usually you cook a tougher meat over low heat for a longer time for the meat to become tender