Unable to change type of FORMAT field despite updating header

[kjaisingh]

I have a VCF with a FORMAT field as follows:

##FORMAT=<ID=EV,Number=1,Type=Integer,Description="Classes of evidence supporting final genotype">

I then update the header line by doing the following:

header = vcf_in.header.copy()
new_header = pysam.VariantHeader()

for line in header.records:
    if line.type == 'FORMAT' and line.get('ID') == EV:
        continue
    new_header.add_line(str(line))
new_header.add_line('##FORMAT=<ID=EV,Number=1,Type=String,Description="Classes of evidence supporting final genotype">')

vcf_out = pysam.VariantFile(args.output_vcf, 'w', header=new_header)

If I now set record.samples[X]['EV'] for a given sample X to a String value, I still get the following error:
TypeError: invalid value for Integer format. This essentially limits my ability to modify values in genotype fields for which I change the type of, though it seems to be a reasonable function to enable.

My thought here is that the evaluation about validity of a genotype field should occur at the time of writing to an output VCF file - which in this case, would be valid, since the output VCF file is built with new_header that has the updated ##FORMAT=<ID=EV tag.