Air Carrier Flight Attendant reported a pressurization leak with the main cabin door inflight. Flight returned to departure airport; flight crew received different aircraft and continued to destination.

Narrative

When I arrived at the gate prior to my report time; the agent asked if I was ready to board on time. I stated I would let her know in the chat after my safety checks were complete. When I got to the aircraft; there was no power. The captain stated there was no GPU and the APU was inoperable. The only ramp agent in the vicinity of the aircraft stated there was no functioning GPU near the aircraft as the closest units were tagged out and her supervisor was in a meeting and unreachable. I communicated in the chat asking the agent to get her lead involved so that it could be communicated to the correct agents that the aircraft needed ground power. I stated I could not board until safety checks were complete. The gate agent still asked when to board. She then came to the aircraft ramp and began yelling at me on the bridge. She was upset about boarding and told me she couldn't talk to the ramp. I could not understand why she appeared angry or why she was yelling at me. I told her to speak to the Captain. The zone lead put in the chat the aircraft was hooked up to ground power as soon as the cart arrived and prior to the aircraft actually having power. As I began my safety checks that require power; a different customer service lead came onto the aircraft and was blocking my access to the equipment I needed to test. I asked him to please move so I could complete my checks; I completed my checks; and communicated that boarding could begin. Passengers boarded; flight departed.After takeoff I heard what sounded like rushing air in the main cabin door area. I continued to listen as it was extremely loud and I tried to determine if it was a faulty air vent or the main cabin door. As soon as the sterile light was off I called the captain and told him about the sound of rushing air coming from the main cabin door. I walked through the cabin and passengers expressed their concern about the extremely loud noise in the cabin. I did not know at the time; but was informed by multiple passengers during deplaning that the gate agent had told them there was an electrical problem with the aircraft while they were waiting to board. I was calmly reassuring passengers when the Captain called me with special procedures. I grabbed the emergency evacuation cards in case the situation changed while the Captain made an announcement informing all passengers we were returning to ZZZ out of an abundance of caution due to an irregularity with the pressurization. I made an announcement to prepare the cabin for a regular landing. I walked throughout the cabin and made sure all passengers were awake and aware we were returning to ZZZ. It was so loud I had to make announcements from the aisle at four locations. Passengers became increasingly panicked. Again; I did not know that they had been told there was an electrical problem by the gate agent prior to landing. I continued listening to the aircraft and walking the aisle calmly; clearly; and confidently telling passengers that like the Captain said we were having a normal landing and were returning out of an abundance of caution; the aircraft was mechanically sound. One person was crying and her breathing had visibly increased. I first told her to look at my eyes and I promised her everything would be ok. I reassured her that her feelings were normal but she needed to remain calm with me because our airplane was just fine and we were landing very soon in ZZZ. Her breathing rate slowed and I got her a box of tissues. I offered to move her seat closer to mine but she declined. I continued to check in with her until we landed in ZZZ. I heard the wheels drop what seemed to be early. The noise in the cabin increased. I heard the PA but had no idea what the Captain had said. From four points in the aisle I announced to passengers the Captain was reassuring us the wheels were down; it was the reason for the increase in noise; this was normal; and we would be landing very soon. I later confirmed with the Captain.As soon as we landed in ZZZ I walked through the cabin making sure all passengers were ok and the cabin appeared as expected. I provided the new gate information as it became available. As passengers deplaned multiple passengers told me the gate agent for the original departure told them there was an electrical problem with the plane; too. Once we landed we switched airplanes and completed the flight to ZZZ1. As passengers deplaned they commented that the gate agent had told them prior to the first departure the gate agent had told them there was an electrical problem with the aircraft; which was untrue. XG19 was my end of duty for the day. XA29 was our scheduled departure; XA38 was the actual out time.