Syncing iPhone with this iTunes library will replace media from another iTunes library - Help please!

Hiya

I am trying to find a way to import an mp4 video onto my iPhone using iTunes. My phone was synced with iTunes on an old laptop previously, but now I back it up to a new PC.

I have imported the video into my library and can see it in the Films section. On the device screen, 'Sync Films' is unticked, but if I tick the box & click apply a window appears with the message "Syncing iPhone with this iTunes library will replace media from anohter iTunes library. An iPhone can only sync with one iTunes library at a time.'

After a search on google, I have seen a similar post and someone advised the following:

"Your personal data, app data photos, etc. will remain on the phone. If your music is all purchased from iTunes you can transfer it to the new computer using iTunes, likewise for apps. What you can't transfer, at least directly, is any non-iTunes music on your phone (ripped CDs, mp3s from other sources, etc). If you have any you will have to load it onto the new computer from the original source. OR get a 3rd party app to transfer it such as TouchCopy.

To keep your data safe, do the following:

• Install iTunes on the new computer.
• Open it and go to the Preferences settings, Devices pane, and check the box to prevent devices from syncing when connected.
• Connect the iPhone to the computer and go to the File menu, Devices, Transfer Purchases From <your device name>. That will copy all Apple content to iTunes from the iPhone onto the new computer.
• Import any other music (non-iTunes) if any to the iTunes Library on the computer (just drag and drop it onto the open iTunes).
• Back up the phone to iTunes.
• Now it will be safe to sync the phone with iTunes."

I have transferred purchases to my library, and can see all my albums, playlists etc in iTunes. I can't seem to find the list of all my apps like I used to be able to, so again googled it & it seems like iTunes no longer has this function.

The bar at the bottom showing data used on the phone is as follows:

• Audio 216 songs
• Photos 4762
• 66 apps
• Documents & data 22.90 GB

If I tick 'Sync Films' the data bar suddenly shows an extra 9GB of data & the sections are as follows:

• Audio 213 songs
• Photos 4762
• Documents & data 22.90gb
• Other 260mb

So if I go ahead and replace the current iTunes library with the one on this PC, I seem to lose 3 songs which I'm not bothered about but also all of my apps! Does anyone have any advice please - I'm too worried to test it and find out the hard way.

Thank you!