+1 to what Docrwm said; also, the K-30 has a built-in interval shooting mode. I've used the same IR remote with both. Weather sealing is a nice feature, but there's no need to replace your existing lenses (unless you begin shooting in much wetter conditions than you already work in). The K-30's higher resolution may make you start thinking about upgrading lenses, but what you have will work fine.
In terms of general photography, especially low light performance, the K-30 runs rings around the istDS (sometimes taunting it, but the DS is too mature to be bothered by such nonsense).