Trans Alt: At or below is on QNH
Trans level: At or above is on Standard (1013.25)
In answer to your question, with a low QNH the trans alt would stay the same. It is printed on the IFR plates as unchangable.
The trans level is given by ATC usually, just for that reason. You can use the bottom 500' for VFR, so if an airway started at FL035, the first available level is FL040, giving a buffer to the IFR traffic, but as it is controlled airspace, you will need a service.
Having a flexable trans level gets over the problem of low pressures, while keeping a fixed trans alt maintains safe terrain sep. You may find that it is several thousand feet above the trans alt, or 500'.
JD