According to the Czech law, if you are obliged to pay child support, you will have to prepare for the possibility that the "going rate" will change over time – and not necessarily in the direction which you'd prefer.
Alimonies, or child support – that is to say, the amount of money which is paid by one parent (who does not have sole custody of the child) to the other parent, so as to underwrite the expenses associated with taking care of the child – are always stipulated by the court – either authoritatively, "by fiat", or in the form of judicial approval of the mutual arrangement reached by the parents. In the former case, the court will always take into account both the actual ability of the obliged party (i.e., the paying parent) to provide support and the justified needs of the beneficiary (i.e., the child).
The amount of child support is being fixed in the beginning, but may be changed by the court if the circumstances change (whether on the part of the obliged party or on the part of the beneficiary). In theory, the amount of support payments may sway in both directions, but in practice, the "adjustment" almost automatically takes the form of an increase of the relevant amount as the child gets older. By contrast, the curtailment of child support payments is a cumbersome process and is rarely seen in practice.
Ceteris paribus, an increase of child support payments is warranted if fundamental changes occur in the child's life as it grows: the first day in school, the shift to senior high school, and thence to higher educational institutions. As these milestones occur, the courts automatically assume increased justified expenses towards child care and thus a need for higher child support. If you want to achieve an upward modification of payments on these grounds, you may essentially wait for time to go by, and then file a motion with the court when "the right moment has come".
Parents who make an amicable arrangement for child support payments ought to keep later upward adjustments in mind and may want to incorporate a relevant clause in their agreement which directly reflects later (increased) needs on the part of the caretaker. Note, though, that even such a contractual arrangement is not binding upon the courts, which may simply ignore it by citing the superior interests of the child. In essence, you are agreeing with the other parent that they won't file a motion for higher child support payments in the first place.
Having said that, there are other situations and circumstances which may result in an increase of the child's legitimate needs (and thus of the amount of child support payments), such as the commencement of costly medical treatment etc. There may also be reasons on the part of the obliged parent – i.e., primarily, an improvement of their financial situation.
Reducing child support payments is in practice conceivable if the obliged parent's income drops substantially, or if that parent becomes obliged to pay support for yet another dependent, or if their financial situation otherwise deteriorates. However, a merely temporary worsening of one's income situation is not enough. If you lose your job and have yet to find another one, this does not mean your child support payments will be curtailed, even if you should for a short period of time be officially listed as an unemployed job seeker with the labor office. The relevant situation must always be a long-term affair. In addition, the courts regularly review whether the parent has not deliberately foregone opportunities to generate earnings. In other words, the court may and will determine what kind of income the obliged parent is actually able to achieve.
As mentioned earlier above, attaining a reduction of child support payments is a demanding process and takes disproportionally longer. Even if the court finally relents and reduces the payments, effective as of the date on which the motion for a curtailment of alimonies was filed, the letter of the law is unforgiving: support payments made and consumed need not be repaid, and any reduction of the amount of child support only ever affects future payments going forward.
If you always duly paid the prescribed amount up until the issuance of a court decision which reduces your payments, then that court decision has only impact on later payments, even if it specifically and expressly applies also to time periods which have already passed – because it is impossible in the current legal environment to enforce the return of previously made support payments. To date, no court has ever ruled such as to interpret the relevant provision in Sec. 923 (2) of the Civil Code differently.
Source: Civil Code (Act No. 89/2012)