Diddy Kong Racing isn't the E-rated game I'm talking about. The first Conker game was Conker's Pocket Tales in 1999 for the Game Boy Color which was rated E. The 2010s games Conker was in were that Project Spark campaign and that HoloLens Young Conker game which were both rated T. I brought that up because Conker is an example of a series that started with an E-rating and later got M-rated games, which disproves your first claim.

Those other decisions about Shadow the Hedgehog were not made at the same time, but at separate points in 2005. The previous cast members were not informed they were being replaced by the Sonic X cast until September at the latest. Sonic's previous voice actor Ryan Drummond claimed he was never asked about the replacement and was only informed until after it went into effect, which means Sega was already recording voice lines with the Sonic X cast for the game and Drummond most likely was not asked to record anything if they didn't ask him about the replacement. Since we do know that Sega decided to censor the script to aim for the E10+ rating after it was introduced in March, the Sonic X cast recording for the game had to have started before the rating was introduced for the unused Shadow swearing takes to happen.

https://www.ign.com/articles/2005/03/02/esrb-introduces-e10-game-rating
https://web.archive.org/web/20151121064154/http://www.sonicstadium.org/2005/09/sonic-voiceover-cast-replaced/

Of course Conker the way he was supposed to've been in Conker's Twelve Tales lives on in Diddy Kong Racing, had no idea he was in any E rated games of the 2010s with how he ended up being. And I could've sworn that the decision for Shadow the Hedgehog to be E10+ was made at the same time the decision to use the 4Kids voice actors in the games was made, both in 2005 when that said ESRB rating was introduced.

Neither of these claims are true. The ESRB does not care about maintaining consistent content ratings between games in a series, that decision rests on the developers and what audience they want to target when designing their games. The ESRB only cares about the content in the individual game they're reviewing. For example, the Conker series has had games with E, T, and M ratings. Even after Conker shifted to a mature audience, developers did not keep the series strictly adult-oriented and tried to utilize it for slightly more family-friendly releases in the 2010's by designing future games to get a lower rating than M. Shadow the Hedgehog was released in November 2005 and is the first game in the series to use the Sonic X cast, not just Jason Griffith, so him recording those swears because Sega wanted a higher rating is possible.

https://www.youtube.com/watch?v=JsmqhQ52p8w#t=273s

Thanks for the additional information to whoever added it in, but there's no way the gaming industry would ever allow there to be any games rated M for Mature in any franchises with games rated E for Everyone. And I don't see how any F Bombs would've been recorded by Jason Griffith, since the dialogue he recorded for Shadow would've been done in 2005 after the decision to use the Sonic X voice actors in the games was made.