So You're Writing a Novel Forum Help with a specific aspect.

My timeline has moved from the past, into the present. If I want to introduce something that happened in the past during a scene from the present, do I have to keep writing past tense? How would I approach this?

The only way I could see you stilll writing in present tense while describing something that happened during the past is through a flashback, but that technique is slightly overused and cliche. 

As far as I know, You should keep to your original tense throughout the entire story. I know I'm far from an expert, but I'd suggest just keeping to the tense you chose at the start.
However, if the situation would necessarily call for it, Your only option would be a flashback.
If you're worried about the time skip while still keeping tense, it should be implied by the reader, and not cause any hiccups.
Hope this helps.

You could always do it through dialogue by having the character who experienced it tell it as a story. That way you can keep the actions in the tense you chose while putting the dialogue in past tense:)