In a recent announcement made by S&P Dow Jones Indices, Uber Technologies, Jabil, and Builders FirstSource are set to join the prestigious S&P 500 index as part of the quarterly rebalance. The news sent Uber's stock, along with the other two entrants, rallying in late trade.

Effective Monday, December 18, Uber, Jabil, and Builders FirstSource will replace Sealed Air, Alaska Air Group, and SolarEdge Technologies in the S&P 500. This change reflects the ongoing evolution of the market and the inclusion of companies deemed to be significant players in their respective industries.

The S&P SmallCap 600 index will also see some reshuffling, with Rambus and Comfort Systems stepping in to replace Jabil and Builders FirstSource in the S&P MidCap 400. On the other hand, Sealed Air, Alaska Air, and SolarEdge will take the place of Orthofix Medical, Rambus, and Comfort Systems in the S&P SmallCap 600.

The market responded positively to these adjustments as traders showed enthusiasm for the new additions. In after-hours trading, Uber's stock saw a significant surge, jumping 5.5%. Similarly, Jabil and Builders FirstSource also experienced gains, with their stocks rising by 3% and nearly 3% respectively.

However, not all stocks fared as well. Sealed Air, Alaska Air, and SolarEdge stocks all retreated by 1%, reflecting a minor setback for these companies.

Despite moving up to the MidCap 400 from the SmallCap 600, RMBS stock and Comfort Systems saw a decline of about 2%. This could be attributed to the fact that more mutual funds and ETFs track the small-cap index, which could potentially impact the trading activity surrounding these stocks.

In the S&P MidCap 400, Houlihan Lokey and Equitable Holdings emerged as strong gainers, while Topgolf Callaway and Vestis showed slight increases. Clearfield stock decreased by 3.4%, and OneSpan fell by 5.4%.

Among the new S&P SmallCap 600 members, ALKS stock experienced a significant rise of 7%. Armstrong World Industries and PJT also gained 6%, while NHC added 4% to its stock.

On the other hand, some companies witnessed a decline in their stock value. Community Health Systems and James River Group saw solid falls, while Invesco and Avid Bioservices experienced slight decreases.

The quarterly rebalance of the S&P 500 and other related indices is an important event in the financial market that reflects the changing landscape of the economy. Including companies like Uber, Jabil, and Builders FirstSource in the S&P 500 is a testament to their growing significance in their respective sectors. Investors will be closely watching how these changes might impact the performance of the indices in the coming months.

The market open on Monday, December 18, will mark the official entry of Uber, Jabil, and Builders FirstSource into the S&P 500, solidifying their positions as key players within the index.