Basically all you need is a good faith belief that a service related disability has gotten worse which typically entitles the Vet to a compensation exam to substantiate this claim. You don’t need to file a formal application instead you can just send a letter stating that you think your disability has become more severe. VA is expected to carefully review all documents “in a liberal manner to identify and adjudicate all reasonably raised claims”. Norris v. West, 12 Vet. App. 413. Claims for disability rating increases are new claims and are not subject to the requirement that a Veteran must submit new and material evidence to a previously denied claim. Theoretically the VA’s duty to assist is triggered by such claims where the VA is to fully develop the claim for increase. The VA must make “reasonable efforts” to assist the vet in locating the evidence. This help includes a medical examination so that the current level of disability can be evaluated.
Usually they rely on the most recent medical information as opposed to older evidence, however the more recent evidence must be adequate for rating purposes. The Vet will also want to support these claims with doctor reports that substantiate the claim including private doctors. If the private dr. report is sufficient for rating purposes then the VA does not need to do its own exam. 38 CFR 3.326(c). Benefits are effective from the date the VA received the claim or the date that entitlement arose, whichever is later. May date back one year prior to the date of a claim for increase if it is “ascertainable that an increase in disability had occurred” within this time frame. 38 USC 5110(b)(2).