I was told if I was putting Flowmasters on my car that The Cadillacs will be changed in the process or bypass is that true

If changing out whole system and not just a cat back system need to retain the catalytic converters[ for emissions and inspections ] if cats not cadillacs are bypassed or blown out will get a code and will fail in most states their emissions and or vehicle inspections and most exhaust shops will not bypass the cats

Not Cadillacs, but catalytic converters. They can be retained and must be kept by Federal law, however some folks trash them anyway. If you keep them on, the car will be much quieter. If you remove them and run just the flowmasters it will be loud enough for the cops to pull you in most jurisdictions. If you are after performance on city streets, try DYNOMAX mufflers for a deep sound but not as loud. Dragstrip only=flowmasters ok.