Can I download an apk of an app I have payed for?
Since you have changed the account from the one from which you purchased the app and are now using a different account , you cannot" use the app purchased. Purchases are tied to the account 

Use of a product purchased in one account in a different account

Add Google Play Apps from Other Google Account?

As You can see in the discussion, you cannot. Google play purchases are tied to the account from which you have purchased and can't be transferred

Google product forums says the same

Second part of your question is not relevant because you can't use the app purchased under a different account