Strongly recommend reading this linked paper, written by CRSP folks:

https://indexes.morningstar.com/insights/analysis/bltcd8e699...

These IPOs will have minuscule impact on the indexes initially. They will have a big impact if they can maintain share price in the first ranking/reconstitution after the lockup period expires.

They will have a big impact if they can maintain share price AND the float increases due to the lockout period expiring (ie. pre-IPO owners selling off shares).

I'd like to know how the CRSP/Morningstar folks feel about the interesting lock-up period rules that Elon has inserted into the SpaceX IPO and how that jives with their analysis.