If ordinary gaseous hydrogen transitions to Rydberg matter or ultra-dense hydrogen, pressure should decrease. The latter two are not gases.
Rydberg matter (H(1)) has been calculated to have a density of about 0.6 Kg/dm3 (8-9 times the density of liquid hydrogen).
Ultra-dense hydrogen (H(0)) is supposed to have a calculated density of about 140 Kg/dm3 and to usually form thin films on metal and metal oxide surfaces.
The conversion to ultra-dense hydrogen alone should be strongly exothermic, releasing a few hundred eV per atom. This however is still well below the realm of nuclear energy.
Once H(1) is formed the transition to H(0) should be spontaneous.