I used bitcoinj library for wallet and it generates 12 word mnemonic seed.
But I made changes in it and created 24 word mnemonic seed.
But I need to convert old 12 word mnemonic seed into new 24 word mnemonic seed.
Please help me with this.
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This is not possible.
The mnemonic words are an encoding of the entropy used in the seed, essentially. A 12 word seed has less entropy than a 24 word one, so you cannot get 24 words out of it. Similarly, you cannot get a 24 word seed to turn into a 12 word one.
Your best bet is to generate a new 24 word seed and move funds over.