Will it be possible to retrieve spot prices per hour, even after the switch to 15-minute MTU?

Official prices from the Market Data API will be provided in 15-minute intervals. However, additional endpoints will be created to offer new price and system price indices in 60-minute and 30-minute intervals for users who need them. This update will be a non-breaking change for existing customers.   

Similarly, a new indices pages will be created in the public Nord Pool market data page and in the Data Portal (under https://data.nordpoolgroup.com/auction/day-ahead/) where these other granularities will be published.