If you really think the 30x25 isn't going to be low enough then putting on a bigger cassette won't be a big deal. I put a lot of climbing miles on the 30x 25 low until I went to the Eastern Sierra Nevada and I borrowed a cassette with a 30 tooth cog and it worked great. It's an Ultegra 10 speed triple and when I put the 30 on the rear I didn't have to adjust or change anything else.
That 30x30 gear helped get my large self up to 10,000 feet.