Where BCDMH tabs have been used for several years, there's going to be a high level of both DMH residue and bromide ions. Chlorine instantaneously converts bromide to bromine or bromate. Conversion to bromate is permanent (more or less) and prevents further bromine 'regeneration'. With pure bromide based 'bromine' pools, bromate formation when chlorine is added makes it possible to end the bromine cycle.

But the DMH apparently inhibits bromate formation, leading to an endless cycle of chlorine => bromide conversion => bromine consumption => conversion to bromide, and then repeat with more chlorine.

Regardless, so long as bromide ions remain in the pool, your pool will be a bromine pool, since any added chlorine will react with the bromide much faster than anything else in the pool. Repeated high levels of added chlorine might eventually convert the bromide to bromate, but I'm only guessing.

BTW, don't drain a liner pool at all, or a concrete pool if the ground water level is high! Doing either one will destroy the pool.