The early step changes (Subsistence to Ag, Ag to Pre-Ind) require even longer time periods than the later changes (potentially 100s of years). The minimum time for a change is really a generation, and given the time frame we have (36 to 53, as we don't support the extended timeframe past then) this is realistically out of reach for anywhere that wasn't effectively fairly well along the pathway anyway. We have some pretty good benchmarks as to what qualifies a nation to achieve a certain technological level, and it is not based on simple "we've got lots of IC" (after all, if China were a single nation in game, it would have more IC than any nation except the 7 major powers, but it is still Agricultural on an overall tech level). You need the educational development of people coming through, growing up with a familiarity of the new technology to really establish that level of technology as fully functional usage for more than a few one-off projects.
Portugal just doesn't deserve anything better than pre-Industrial, however you slice it. Spain is a big economy, but it's performance in the heavy industry field is very lacking, which is why it is only pre-Industrial. Finland suffers from being small, amongst other things, although it is better than Portugal, certainly. NB "Pre-Industrial" is perhaps a slightly misleading term - I think the original name was "Early Industrial", which probably gives you a better flavour of things, but "Pre-Industrial" was the name that got used in the released version.
The other thing to consider is that there are actually advantages to being lower tech - you get cheaper costs for many units, cheaper resource costs to run your IC (though you don't directly see this - it is built in to the resources given to each province, since we can't change the 2 Energy / 1 Metal / 0.5 Rare per IC cost via techs). Whilst AoD may be more designed as a sandbox game - CORE is not. You are not meant to be playing Bhutan and building battleships with it in CORE, even if you might be able to do so in vanilla.
This doesn't mean that there won't ever be a way to move up a tech level in CORE. It does mean it will be very hard and (frankly) not likely to be worth the effort in the timescale of the game, although I do understand how some people see it as a goal in itself, as well as potentially playing with a later end date.
Tim