Only those countries that are on the OECD DAC list are eligible to receive ODA funding. OECD, based on per capita GNI, compiles the DAC list. The list is up-dated every three years. If per capita income in a country increases and remains high, that country may move up from one income category of the list to the next and eventually graduate from it (as for example Chile, Seychelles and Uruguay graduated in 2018). Equally, if per capita income in a country falls then they will be added to the list (as South Sudan in 2011).
It is important to note that just because work in a particular country has been either eligible or ineligible in the past, this does not mean that will continue to be the case.