Swtiching current accounts

I’m thinking about using Monzo as my current account, mostly because it gives a nice itemized breakdown of different types of payments going in and out of the account. I’ve seen that there’s a current account switching service which I could use to move things over from my current current account. I understand that if I use this service, any stray payments which got sent to my old account would be forwarded to my new current account for three years after the old one was closed.

I have a couple of questions related to switching;

• If stray payments to the old account get forwarded for three years, would I be notified when this happens?

I imagine a scenario where my friend has my old bank details saved on his online banking, and sends some money over to me within three years of the switch. This gets forwarded to my new account, but I’m not notified that it happened. So then I don’t know to tell him to update the old details that he has, and he sends me some more money to the old account details four years after the switch. Now the money gets lost somewhere and it’s a hassle to get it back.

If this is the case, it seems to me like it would be better to leave the old current account open for some extended period of time and forward any payments myself. Then I would be able to notify anyone who had my old bank details. This leads on to my next question;

• Is there any downside to having additional current accounts open?

It seems to me like I could just have both accounts open, no problems. Would this negatively affect my credit score or something like that?