Constitution Modifier Increase and Hit Points

[Major Doom]

If a character's, or animal companion's, Constitution modifier increases due to gaining a level and spending the Ability Score increase on Con, for example from 19 (+4) to 20 (+5), does the Con modifier change affect Hit Points just for the new level or is it retroactively increased from first level and on?

[Jiggy]

This sounds familiar.

[Are]

The hit points are increased for all levels.

[Major Doom]

Thanks for the information.