A compressible random-phase approximation (cRPA) study is performed for a molten UCOT diblock copolymer of incompatible pairs to suggest an effective Flory-type interaction parameter chi(cRPA) through mean-field spinodals. The compressible nature of a given copolymer system is carried by chi(cRPA), which is further divided into chi(app) and chi(comp), where the former stands for the unfavorable exchange energy density and the latter contains the effects of compressibility difference between blocks. It is shown that chi(app) and chi(comp) upon pressurization contribute to phase stability in a reverse way to each other. The balance between those two chi's is found to yield the diversified behavior of microphase transitions under pressure for UCOT diblock copolymer melts. The comparison of the theory is made with the experimental phase behavior at different pressures for a typical UCOT system from polystyrene and polyisoprene. It is revealed that the microphase transitions and their response to pressure in the copolymer are mostly driven by chi(app). A Landau free energy combined with the cRPA in the case of chi(comp) approximate to 0 is also suggested for the copolymer to yield a reasonable prediction of the pressure coefficients of the transition temperatures.