Q: How do I make purchases in apps I bought with a different Apple ID?

Here is the scenario:

So Minecraft PE have finally added texture packs, to the new Minecraft PE 15.0

The only problem is, it costs money.

Now I am happy to pay, but whenever I try to pay it tells me "This app has been bought with a different Apple ID."

I Have three questions, if you answer one of them, the other ones do not need to be answered. Reason being answering one of them answers the question, eliminating the need for my other questions.

Here is what need to be answered:

1: How do I pay for a purchase in an app, without using the Apple ID I used to download it?

2. How do I delete and reinstall Minecraft, without deleting any of its content? (Worlds)

3. How do I send money to a different Apple ID?

To Justify:

The second one is more of a question I should be asking on the Minecraft realms, but you someone here might know.

Also, I do have access to the Apple ID, but I'm not going to pay using more money.

I Have iTunes money on the Apple ID I want to send money from, but not credit card money.