thanks a lot for your insights. I really appreciate the kind help in this forum. Unfortunately, I only have 4 measurement points per person (over two years). In fact, I tried to reproduce the approach in a paper (Maia et al., 2016, JOB) with a similar setting (3 time points over three years) that says:
"To test hypotheses, we calculated the value of the latent variable of change for each individual using LGM. We then partitioned the sample into three groups: individuals with negative, positive, and zero time parameters. [...] Zero time parameters were established while assuming a 98 per cent confidence interval around zero. Note that this is a rather strict assumption and implies a conservative testing of our hypotheses."
That is why I have tried to establish the confidence intervals around the individual slopes. But it seems to me that I don't have enough variance in the slopes between subjects, or the author meant something different with "98 per cent confidence interval around zero".
Either way, I agree with you that this is probably not the most common procedure. Nevertheless I liked it, so I tried to replicate it with my data, but so far I haven't been very successful :)