I had to go to the US consulate last year to obtain a 10 year visa to travel to the USA, i have recently had another conviction against me - do i need to declare this new conviction?
If it's for an offence involving 'moral turpitude' (e.g. theft) you'll almost certainly be refused a visa with such a recent conviction. (The best you could hope for would be that your application would be treated as seeking a 'waiver of permanent ineligibility'. You'd have to wait many months before hearing whether you'd been granted the waiver).
If the conviction is for an offence not involving moral turpitude (e.g. most types of assault) you might stand a chance of getting a visa but I still wouldn't be too hopeful.
As an example, someone posted on here to report that her husband had to wait 14 months to see if he could get a US visa. He'd been convicted of two counts of driving without insurance. His application was refused.
At renewal - yes. The complete criminal record must be declared.
But visas are not normally just for one trip. They are valid for multiple entries for up to 10 years and during that 10 years it would not be normal to be asked about a criminal record on arrival in the US.
So there's still the question - does a <<new>> conviction invalidate an <<existing>> visa?
The answer seems to be that it doesn't invalidate the visa as such. What it does do (assuming the wrong sort of conviction) is makes the visa holder ineligible to enter the US, as a result of which the US authorities will cancel the visa when they find out. But until they cancel it it's valid.