Microsoft will double the per-PC price of support for enterprises still stuck with Windows XP systems when the anniversary of the aged OS's retirement rolls around in April, a licensing expert said today.

The per-PC price for what Microsoft calls "custom support agreements" (CSAs) will jump to $400, the expert said after requesting anonymity.

CSAs provide critical security updates for an operating system that's been officially retired, as Windows XP was on April 8, 2014. CSAs are negotiated on a company-by-company basis and also require that an organization has adopted a top-tier support plan, dubbed Premier Support, offered by Microsoft.

The CSA failsafe lets companies pay for security patches beyond the normal support lifespan while they finish their migrations to newer editions of Windows. Most enterprises have shifted -- and are continuing to do so -- to Windows 7 rather than adopt Windows 8.1.

Last year, just days before Microsoft retired Windows XP, the company slashed the price of CSAs to $200-per-device with a cap of $250,000.

Because a CSA is an annual-only program -- and Microsoft limits each organization to just three years of post-retirement support -- agreements must be renewed each year. The first renewals come due in less than two months.

Ideally, companies that signed up for a CSA last year will have retired large numbers of Windows XP machines in the interim. If a firm reduced the number of Windows XP PCs by half, it will pay the same as last year if it renews the agreement at the higher per-device price.