Plenty of people have printed hundreds of flyers, or a poster for a shop window, only to find months later that the code leads to a page asking them to subscribe.
Many free generators produce what they call a dynamic code. It does not contain your link at all. It contains a short address on their server, which forwards to you, and that forwarding is the product they are selling. When the free tier ends, the code you printed is theirs, not yours.
A static code holds your address directly. There is no middle step to switch off, nothing to count, and nobody to keep paying. The trade is that it cannot be re-pointed later, so use a link you control, and redirect on your own site if the destination ever moves.
The easy way to tell which one you have is to scan it and look at what appears before the page loads. If it shows a domain you do not recognise, the code belongs to somebody else.
These are static codes. The address is written into the pattern, so the code works for as long as the page it points to exists. That means the link needs to be somewhere you control and intend to keep.
If you might move the page later, point the code at an address on your own site that you can redirect, rather than at the final page directly. A code that holds plain text or a contact card needs no web page at all, and so it can never stop working.
Not in the code. Point it at an address on your own site and change the redirect there, which gives you the same flexibility without depending on anyone.
More text needs a bigger grid. Keep the address short, and raise error correction only as far as you actually need.
PNG for a screen, SVG for print. A vector has no fixed resolution, so it is sharp on a business card and on a banner.
The code will still point to the old address, because a static code cannot be edited. Keep the old address alive or redirect it to the new one.