Biennial elections are being held for 58 Rajya Sabha seats across several states and by-poll for one in Kerala.
"We realised that we don't have the required numbers to form the government and we don't want to indulge in horse-trading," Fadnavis told reporters after announcing his decision.
Adityanath will be the 21st UP chief minister and the fourth from the BJP.
The theme of the reshuffle is to fulfill Modi's earlier promise of 'maximum governance, minimum government.' Some key ministries are likely to be merged.
The AgustaWestland issue was fiercely debated in the Rajya Sabha on Wednesday with members of the Bharatiya Janata Party and Congress blaming each other.
The Bharat Ratna conferred on Madan Mohan Malaviya has exposed the frictions within his family, reports Manavi Kapur