Can 25mL of 10% phosphoric acid substitute for 10mL of 25% solution?
Not reliably. Acid strengths measured in percentages are usually given as percent by weight, and if that's the case just swapping the volume numbers around will not provide you an accurate quantity of acid.

You first need to be sure of how the percentages are expressed.  Assuming they are percents by weight, you would then need to look up the density of phosphoric acid at the two percentages and work out the calculation so that you were adding the same number of moles of $\ce{H3PO4}$.