The modification process is a bit more streamlined than the divorce process, mainly because there is a lot less discovery. Since the property division portion of a divorce agreement or judgement is not modifiable, issues of valuation are rarely necessary.
Modification has a defined starting date: the date of the divorce. What were the circumstances then? This is shown by the financial statements or the custody plan as well as the court judgement or the parties’ agreement. Then the question becomes: What are the circumstances now and what is the material change in circumstances that warrants a modification?
If the modification is being brought under the new alimony law, then the change in circumstances will have been set forth in the law itself. In these cases, generally the person paying is asking for a reduction. The burden of proof is on the recipient to show why (if she/he can) the reduction or elimination should not take place.
Aside from alimony cases, the party alleging the change in circumstances usually has the burden of proving that a material change in circumstances has occurred. Most judges will not allow modifications at the time of motions for temporary orders absent really extraordinary or emergency circumstances. These motions follow the same procedures as in a divorce.
As in divorce there will be a pretrial conference, and if you are unable to settle, eventually a trial. I have not seen any statistics, percentage wise, on how many cases of modification actually go to trial, but I would not be surprised if it were fewer than divorces because in so many cases the pretrial issues are narrow enough for a judge to give a clear answer.