During my last discussion of this with some of my old players from the old saga, who'lll also joind th new saga, we reached a verdict.
Since things get complicated by the fact that 2 out of 3-4 old players (and 1 new) wanted to convert, we needed the balance. Had we all been converting, it would have been easy.
One player suggested I try and create my old character according to the 5th ed rules, and see how it went points-wise, and advance her these 30 years out of apprenticeship. Of course, I blew the budget tremendously, because not only was 4th ed progression steeper than 5th, our saga had ample good books and vis, plus my Maga had most exp-boosting Virtues to help her along.
At first, I cut away all items she had created, which no longer adhered to the rules; she had a lot of characteristic boosters, which now require a lot higher levels. She is a Flambeau Hoplite, and has a lot of combat/missile spells, where the essentially same spell is known at different ranges (Reach, Near, Far). This would actually be a good idea in 5th ed, due to the new (and might I add, brilliant) Penetration rules. But said ranges are all Voice now, so...
Many Abilities had reached too high levels, due to some Exposure House Rules of ours, and quite a bit of luck. And these do no longer serve their original purpose. Concentration does not add to bonus Exp when Reading. Finesse is not used as extensively, when missile spells (which are Resisted) no longer needs to be Aimed. Our Exposure rules back then actually causes imbalance. Since the scale for Abilities is 5 times as detailed in 5th compared to 4th, getting 1 exp Exposure as a side-benefit in a season of other projects in 4th actually converts to 5 exp in 5th. This is quite a leap, since Exposure gived 2 ex IIRC. A lot of her Abilities no longer existed - she had a 4th ed Affinity at a high level (but Focus is soooo much better), and cutting them too also made up for the deficit.
Finally, the Arts got some serious cuts, because of the low, low efficiency of Vis Study in 5th ed compared to 4th. This was not helped by a House Rule back then, that added Lab Quality to Vis Study, and rules expanding study beyond the 1-3 pawns. You could gain from a 4th pawn of vis, but it cost 2 (so you study from 4 but use 5), and even a 5th, but that one cost 3 (study from 5, use 8). Luckily, cutting Arts at very high levels, raoidly reduces te deficit. As mentioned by another poster, the numbers don't matter as much. What i was really interested in, was to not start from scratch again. My Maga had acheived a lot, researched, studied, made a Talisman, Bonded a Familiar, achieved status as Hoplite and was just starting to teach 2 apprentices at the same time. She was involved in a plot with the re-appearance of Diedne, and an evil alliance between them and Dark Faeries, waging war om Hermetic magi and Mundanes/Church alike, catching the order between the proverbial rock and a hard place.
Virtues and Flaws were quite easy to convert, but some changes were made, but the concept, background and personality unchanged in essence.
But I managed to fiddle the numbers, without sacrificing any concept and the important stuff (the non-mechanical stuff). I ended up only 6% over budget. Taking into account how many stories she participated in, and the resulting exp, various beneficial Twillight effects and bonus achieved by role-playing, this is not much off. Mind yiu, she does have more Flaws from Twillight than beneficial effects. And her Warping is higher than the standard years-as-magusx2, since she started on Longevity quite early. She is 52 years old, but still looks 19, due to a good Longevity, Strong Fae blood and good living conditions.
Hope this can actually help or enlighten others.
We still have another 4th ed maga - a Quaesitor - that needs loking at. But as she spent a lot of time on Quaesitor duty rather than in the Lab or library, and wasn't as boosted as my Flambeau, it should be even less difficult to convert her.