Moving from casual 10-man content to the 25-man content is an uphill battle. It can be done, but takes the commitment of more than just one person. 25s can be done in a casual manner. Several guilds on my server do it well. Get everyone on the same page. Lay out a plan for how you want to reach 25-man and set measurable goals. Share these goals with the guild!
Are your weekly raid scheduled for the same time/night every week? This is the first step. Make things consistent so that people can show up the same time every week and expect something to happen. Then give them something to do at that time. Not enough for kara? Run a heroic. Too many for kara? Run kara and a heroic. PUG if you have to, but make sure people are busy for the scheduled raid time and spread the word. Once people realize that something, anything will be happening they will show up. Consistency is key. Be upfront with your goals and raid times when recruiting.
Provide incentives to your current raiders to convince them to stick around. Give them a vested interest in the future of the guild. Delegate your responsibilities. Responsibility leads to interdependence which leads to cohesion. Cohesion leads to ownership. When guild members believe that they own the guild, they move en masse towards a common goal (the one you laid forth ealier!). Suddenly, you have transformed your job from cat herder to coach.
Throughout all of this, maintain a positive attitude and keep everyone's spirit up. Muggs stated above, "...they vastly outgear Gruul's now thanks to the badge gear, so it's really just a matter of convincing them of that." This is 100% true! Even without much kara/badge gear, you can destroy Gruul & Co. If people believe they can do it and they will. /train
Rabid Wombat gets +2/+2 for each creature enchantment on it.