The promotion that allows new users to try Xbox Game Pass Ultimate for a month for one euro is only one side of the coin, since this same offer also offers the option to existing customers to upgrade their current Xbox Live Gold subscription and PC Game Pass to an Ultimate subscription. This system is very beneficial, since all the remaining time of the update is converted to Xbox Game Pass Ultimate. Until now the ratio was 1:1, but coinciding with the increase in the price of the service and the return of the promotion, the conversion rate has changed to a less profitable ratio of 3:2.
For example, a user could accumulate three years of Xbox Live Gold and then pay one euro to convert it into three years of Xbox Game Pass Ultimate, whose subscription is much more expensive. Now that Microsoft has brought back the one euro promotion, this option is available again, but due to the price increase the conversion becomes 3:2 instead of 1:1. That is, 36 months of Xbox Live Gold would become 24 months of Xbox Game Pass Ultimate.
Below you can see the conversion rate (subject to change) that Microsoft provides in the Xbox Game Pass Ultimate FAQ.
The Xbox Game Pass Ultimate thread on the Xbox Series > Online forum is discussing the new conversion rate and the best options.