They can be implemented during the season but it will be very slow because scheduling means that there isn't actually many full training sessions, most of the sessions are focused on passive recovery and installing gameplans that are within a tactical framework that was set during summer. And no in normal circumstances they are not really practicing new tactics or implementing new tactics during the season, a manager can decide to still do it but it will lead to significant inconsistency which is what we are seeing and what Liverpool or even Arsenal saw under Klopp and Arteta.
To give you a clear example, during preseason you have several hours per day that you can allocate to tactical drills with the entire squad during the season you maybe have 90 minutes per week or every two weeks and it is almost guaranteed that it won't be with the full squad. And those in season sessions do not include much actual field implementations unless you want to destroy your players conditioning.
There are two aspects of conditioning that makes it nearly impossible to switch quickly, the first one is the season planning, players are trained to physically peak at a certain period during the season and this process starts during preseason, the other thing is that the quicker you want the change to happen the more intense the training has to be which is a big source of fatigue, in the short term it leads to worse performances and injuries especially when you have to play competitive games twice a week.
Basically there is no shortcut on these things, you need time for both of these things and time isn't something that you have during the season, unless you really don't care about results.