I assume this is for a mod. If the background changes are limited to sub-stage changes, then it should be possible with hex editing (from what I remember). As in if you want different background for 1-1 and 1-2 then it should be fine. If you look back far enough, I think me and YinYin released something that did that (boss mode?).
If you want it within the same sub-stage you need to do what YinYin suggested.
If you want it within the same sub-stage you need to do what YinYin suggested.
10 ʏᴇᴀʀs sɪɴᴄᴇ ɪʀᴄ ɢᴏᴏᴅ.ɪ ᴡᴀʟᴋ ᴛʜʀᴏᴜɢʜ ᴛʜᴇ ᴇᴍᴘᴛʏ sᴛʀᴇᴇᴛs ᴛʀʏɪɴɢ ᴛᴏ ᴛʜɪɴᴋ ᴏғ sᴏᴍᴇᴛʜɪɴɢ ᴇʟsᴇ ʙᴜᴛ ᴍʏ ᴘᴀᴛʜ ᴀʟᴡᴀʏs ʟᴇᴀᴅs ᴛᴏ ᴛʜᴇ ɪʀᴄ. ɪ sᴛᴀʀᴇ ᴀᴛ ᴛʜᴇ sᴄʀᴇᴇɴ ғᴏʀ ʜᴏᴜʀs ᴀɴᴅ ᴛʀʏ ᴛᴏ sᴜᴍᴍᴏɴ ᴛʜᴇ ɢᴏᴏᴅ ɪʀᴄ. ɪ ᴡᴀᴛᴄʜ ᴏᴛʜᴇʀ ɪʀᴄ ᴄʜᴀɴɴᴇʟs ʙᴜᴛ ɪᴛ ɪs ɴᴏ ɢᴏᴏᴅ. ɪ ᴘᴇsᴛᴇʀ ᴢᴏʀᴛ ᴀɴᴅ ᴛʀʏ ᴛᴏ ʀᴇsɪsᴛ ʜɪs sᴇxɪɴᴇss ʙᴜᴛ ɪᴛ ɪs ᴀʟʟ ᴍᴇᴀɴɪɴɢʟᴇss. ᴛʜᴇ ᴇɴᴅ ɪs ɴᴇᴀʀ.ɪ ᴛʜᴇɴ ᴜsᴜᴀʟʟʏ ʀᴇᴀᴅ sᴏᴍᴇ ᴏʟᴅ ɪʀᴄ ʟᴏɢs ᴀɴᴅ ᴄʀʏ ᴍʏsᴇʟғ ᴛᴏ sʟᴇᴇᴘ.