I want to know what kind of contract UFC fighters have signed that gives UFC the power to change the contract whenever they feel like it and make them wear uniforms
All it requires is a standard approval clause. Here is the boilerplate clause that they use for attire and advertiser approval per the Eddie Alvarez contract.
21.1 Fighter covenants and agrees that no wording, symbols, pictures, designs, names or other advertising or informational material (i) for any beer, alcohol, beverage company, tobacco, casino or gaming company, media company (including, but not limited to, HBO, ABC, SNI, ESPN, Spike, any Viacom owned network and any Internet related company); (ii) of any sponsor in conflict or competition with ZUFFA or any of ZUFFA's sponsors; (iii) of any sponsor causing injury to the reputation of ZUFFA or ZUFFA's sponsors or their respective officers and owners; or (iv) which has not been pre-approved in writing by ZUFFA shall appear on the trunks, gloves, robe, shoes or any other part of the costume or the body (including by use of temporary or henna tattoos) of Fighter or any of Fighter's Affiliates during or at any Bouts, Pre-Bout Events or Post-Bout Events. For the avoidance of doubt, all sponsorship and endorsement approvals shall be at ZUFFA's sole discretion. Fighter further covenants and agrees to only use the gloves approved and supplied by ZUFFA.
21.2 Notwithstanding the foregoing, ZUFFA shall have the right to include any advertising or other informational material of ZUFFA, ZUFFA's sponsors or other entities designated by ZUFFA on the trunks, gloves, robe, shoes, regalia or any other part of the costume of Fighter or any of Fighter's Affiliates during or at any Bouts, Pre-Bout Events or Post-Bout Events.
https://docs.google.com/file/d/0B4Tw9Kwt3agyc1MwRDVFZ05EY0U/edit?pli=1
In other words, ZUFFA fully defines that they have the right to both define which advertisers can be on every piece of clothing as well refuse any piece of clothing that has advertisers that they don't want to see on that clothing.
And since every fighter signs that contract, then it's up to ZUFFA whether they want to let you rent out that space (assign the rights), or not rent out that space (reclaim the rights). And they can do so arbitrarily and at their own discretion.
It's similar to me letting you temporarily park your car in my driveway. If I have you sign a contract that lets me revoke your right to use that space at my discretion, it's up to me whether I let you put a sign on the side of your car, or I can simply tell you that I'm sick of seeing that sign and if you want to keep using my driveway the sign has to go.