Is it permissible to change the intention of a voluntary prayer from four rak'ahs to two rak'ahs after commencing it, in the event that congregational prayer is established and there is no fear of missing it?

The permissibility of changing one's intention for a specific number of rak'ahs (units of prayer) is a matter of scholarly disagreement, with prohibition being the more cautious approach. So, if you intended four rak'ahs and then the congregational prayer began, complete your prayer as four rak'ahs if you do not fear missing the congregation. If you do fear missing it, then discontinue your prayer. You are not permitted to change your intention to two rak'ahs, as congregational prayer takes precedence. However, for those who permit changing the intention, the matter is clear.