You can just swap the servo but you're only going to get about 4.5 volts or so of power to the servo that way, which will not give you all the torque that the servo can provide. If you add a Castle BEC and an upgraded servo, the BEC can be programmed to the correct voltage that the servo requires. The Hitec 7955 is a 6.0v servo and the 7950 can run at 7.4v. The Castle BEC is about $25 and if you order it from Matt at Anderson RC in Thomasville, he can program the BEC to the correct voltage that you need before he ships it out to you. Otherwise, you will have to buy the Castle Link yourself or find someone that has it that can program the BEC for you. I always just had Matt program my BEC's when I bought them.
How much $$ are you willing to spend on a new servo? That will determine which servo you can get, the 7955 is about $125 and the 7950 is about $150 plus the $25 for the BEC.
The RC crawlers are alot of fun but you can spend alot of $$ on them very quickly. I started out with a brand new Night Crawler and it was a very fun crawler with just a few simple upgrades. Then that snowballed into a full-on 2.2 Sportsman class (shaft driven, single motor, no dig) build plus I also bought an Axial XR10 when they first came out and built it up into an all-out 2.2 Pro Class rig.
