We need to know what else you are changing, Thaumcraft is pretty stable but when you change your PLAYERNAME.thaum file to one that is from a version before 4.2.2.0 your scanned items will mostly disappear but your research should stay the same. Are you adding in or removing other Thaumcraft addon mods, things like Automagy or Forbidden Magic? Are you replacing that file while your character is in the world? Do you keep restoring that month old backup file?
To answer your question I do not know what will happen with a 4.2.3.4 version PLAYERNAME.thaum file loaded into a world running Thaumcraft 4.2.1.4, so rolling back to 1.0.3 might require using that month old backup file again.
OK to be clear -
1) I set my loader to 1.0.4 once and have left it there, days later my reasearch was found to be missing ( I don't use Thaumcraft every time i play minecraft so I found the problem when I next used Thaumcraft)
2) "We need to know what else you are changing" - the ONLY time I change anything is when I look at my research table or check for the aspects to display in my inventory and find they have reset - meaning 1 of each base aspect in the research table and no items showing as being scanned (so no aspects displayed in my inventory) and my research book research being lost
3) when my aspects have been reset to zero is when I copy an old PLAYERNAME.THAUM file - it appears what you are saying is that "when you change your PLAYERNAME.thaum file to one that is from a version before 4.2.2.0 your scanned items will mostly disappear" - or to put it another way MY attempt to resolve the problem of my research disappearing is causing my research to disappear causing me to try to resolve the problem which will of course cause my research to disappear and on and on in an endless loop.
If only there was a way to have my research not disappear - the effect of this would be that I would not need to copy an old PLAYERNAME.THAUM file to get them back.
4) Nothing has been added, I have removed journeymap otherwise DW20 1.7.10 out of the box
5) Not copying while character is in game - mostly exited completely (No Minecraft no FTB running), occasionally have minecraft launcher started but no game loaded.
6) I am (restoring the "month old" as I can not find a more recent version that gives the required result ie reasearch restored to existing
If I sound frustrated it is because I am, after my last post I set about getting the items I need for turning into essence into 14 labels on the wall so I could easily break the labels dropping the 3 items and scan them every time I restart minecraft, not ideal but without another solution worth a shot. Unfortunately my computer crashed (after I completed this and before I made a backup) and I lost all that