** Casey's General Stores CASY.O shares up 0.3% at $750 premarket as co set to join S&P 500 .SPX this week

** Convenience store and gas station operator to replace Hologic HOLX.O in SPX effective Thurs, Apr 9, S&P Dow Jones Indices announced late Mon

** Medical device firm HOLX being acquired by affiliates of Blackstone BX.N and TPG TPG.O in $18 bln deal expected to be completed on or about Tues

** In turn, cloud platform DigitalOcean Holdings DOCN.N to take CASY's place in S&P MidCap 400 .IDX. DOCN shares slipping 1.5% before the bell

** REIT Broadstone Net Lease BNL.N to replace DOCN in S&P SmallCap 600 .SPCY. BNL stock rising >3% premarket

** Firms entering the SPX typically see their stocks climb as actively managed funds benchmarking the index buy shares

** Stephens analyst Melissa Roberts says stocks moving from MidCap to SPX historically lagged modestly by an average 0.6% from announcement to migration, but then outperformed by an average 3.2% in three months post-migration

** Iowa-based CASY's shares have risen previous four sessions, notching record high close of $747.77 on Mon, giving co ~$27.6 bln market cap. Stock up 35% YTD and 80% over the past 12 months at close

** 11 of 19 analysts rate the stock "strong buy" or "buy", 8 "hold"; median PT $744.50, per LSEG