Best Way to replace new footage with old footage when they have different start/end times?

[defaultceq9oemxoa79]

Let me try to explain my situation. My boss has given me an old Premiere file where two source clips (Parts 1 and 2 of the same livecast replay) have been chopped up, keyframed, and geneally heavily edited into a timeline. The source footage was low-quality, so he's sent me higher-res versions of parts 1 and 2 and asked me to essentially replace the footage in the timeline with the new stuff, keeping any cuts/adjustments/edits intact.

Main problem comes with both pairs of footage having different start and end times (all consisting of studio prep or a "starting soon" broadcast titlecard, nothing that will appear in the final delivery). I've synced the footage up in a timeline and I've ensured the resolution/frame rate/audio sync/pushin is all matched (screenshot included for clarity sake), my main dillemmas involve how to safely insert the new footage into the heavily-jumbled old timeline. I can't do a 1-1 "replace with clip from bin" as the differing start times will cause all the timings to break.

My opening ideas:

1. Add blank space to the high-def footage the exact length of the missing opening segments, export that into a fresh video, and then 'replace with clip from bin' that new export. My only concern is whether exporting the footage an extra time would lead to quality loss in any way.
2. Nesting the high-def footage with the right amount of opening blank space and then somehow replacing the low-def footage with the nest. Is that even possible, "replace with nest from bin"?
3. Possibly working with the "Render and Replace" feature? I'm not 100% sure I'd be using this feature properly, but it does leave a new piece of footage in the project I can "replace with clip from bin". I'm not terribly familiar with this feature, am I going in the right direction at all with this train of thought?
4. Of course, if all else fails I can go through the export timeline and painstakingly replace each snippet of footage by hand, carefully recreating any alterations, but I'm hoping to avoid that. 

Any advice or suggestions would be super appreciated. I'm happy to answer any followup questions if my situation was not adequately or clearly explained.

[Carlos Ziade]

First of all, BEFORE DOING ANYTHING, DO A BACKUP.

Save another version of your project, something like <project_name_02>

If your boss has done a proxy workflow, you can select all clips in Project Panel,

right click and relink with original, that if the high res footage preserve the same

naming as the proxies (i.e: proxy workflow).

If it is not a proxy workflow, you can try to offline all source clips in the project panel,

then relink one of them to the high res version of it, hopefully, again if the clips naming

is the same on both low and high res footage, and if one of the offline clips is pointed to

the other folder where the high res ones are, it will relink all others to the high res verions.

Otherwise you will have to replace each source clip in the project panel with the high res 

version of it.

You can also import the high res footage to your project, select each edited clip on the timeline,

then Alt+drag the high res source clip and drop it over the edited clip on the timeline, this will

hopefully preserve all keyframes, cuts and effects.

For nested stuff, you can go for a pancake workflow for ease of editing.

It is very important when working with low res footage and planning to replace

with high res for final delivery, to go for a proxy workflow.

If nothing works, you will have to share a screen recording on what is 

exactly happening and how your project is organized, so that we can 

help you better.

[Ann Bens]

I would make the new files exactly the same as the old ones and do a Replace Footage in the Project window.