In order to substitute lawfully, the 2 drugs must be bioequivalent and in the same dosage form. Therefore the 2 drugs not only must be chemically equivalent, but also therapeutically equivalent. In your situation, dextroamphetamine and dextroamphetamine salts combo are 2 different drugs. The former is a single ingredient product while the latter is a mixture of salts.
The basis of bioequivalence is determined by the kinetics of absorption, distribution, metabolism and elimination from the body. In general, a drug and its salt form will have different dissolution rates and therefore will be absorbed at dfferent rates, distributed differently, metabolized differently and eliminated differently from the body. So they have different effects on the body. In most cases, the drug itself and its salt are similar enough that there is no major therapeutic difference and are considered bioequivalent. However it’s not true in all cases.
A controlled release formulation (such as sustained release, extended release, delayed release, long-acting, etc.) releases the drug slower, get absorbed slower and reaches the peak plasma level slower than the regular formulation. Therefore the two formulations are not bioequivalent and are not substitutable. The d-amphetamine salts combo prescription you have now may be an extended release formulation and so the effects are very different from the single ingredient d-amphetamine that you had before. The prescriber might have changed it.
Another example is the drug hydroxyzine. It is marketed as the hydrochloride salt under the trade name Atarax® and is also marketed as the pamoate ester under the trade name Vistaril®. The salt form cannot cross the blood brain barrier, so it is used peripherally as an anti-histamine. The ester can cross the blood brain barrier and acts centrally as a minor tranquilizer. These 2 drugs are not bioequivalent. In this case, the 2 drugs distribute dffferently and are not substitutable.
For more information on bioequivalence, check out The Orange Book published by the FDA.