Great question — answering as the UphillTech team:
50 PSI is way too much for a typical MTB. They will be able to recommend you a better value at the shop or you can use one of the online calculators, such as the one by SILCA . If they are replacing the fork for you for a new one, they can surely put the stem higher. Tell them in advance though, otherwise they might cut the steerer too short according to your current position. A stem of a different length might be in order to account for the change in the distance of the handlebars.
Full answer by Vladimir F Героям слава: https://bicycles.stackexchange.com/a/96295 (CC BY-SA 4.0).
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