After little-bit of homework I found one way of doing this. I am posting this way if anyone could get help in future.

Extracted the header by

bcftools view --header-only raw_variants.vcf | sed 's/##contig=<ID=/##contig=<ID=chr/' | sed 's/##contig=<ID=chrMT/##contig=<ID=chrM/' > hello.vcf

Followed by

hello.vcf -> hello

Edited the name accordingly and run:

tabix -r hello raw_variants.vcf > raw7.vcf.gz

or

bcftools reheader -h hello raw_variants.vcf > output.vcf

Works well for me.