The Tangent 500 is slightly larger (taller) than the 400. It uses a passive radiator instead of ports as in the 400. I would stick with what you've got if it were me. The 400 has slightly less power handling, misses 3 Hz on the bottom that the 500 has. I wouldn't think it is worth the trouble to change. They both use Heresy II parts.
Bruce
'86 LaScalas (LS-BLS w/s) w/BEC tweeters and DHA2 crossovers, '89 Heresy IIs, '72 JBL 4311s, JMA Merlin Pre, Welborne Labs 2A3 Moondogs, Former Dynaco ST-70, H/K 430 x 2