Will makes a good point about action.

However, if your guitar is well set up for lights (say 12-54) in standard tuning, a set of mediums (13-56) with three of the strings at reduced tension for DADGAD shouldn't present a huge difference in action, if any noticeable difference at all - on my Dread, I have mediums (13-56) which I tune standard, Drop D, Double Drp-D and DADGAD without any noticeable effect on action-height. Likewise the lights (12-54) on the OM.

It's always a good idea to know the maximum and minimum gauges recommended by the builder of your instrument - if you stay within those parameters, the likelihood of string-tension causing damage to the guitar is minimal.

Usual disclaimers - IMHO, YMMV, IOSBTBDO, etc.