I mean that by lowering your Lantus, you have less insulin in you. Lantus gives you a steady amount of insulin in your system over 24 hours. When you eat and give yourself Novorapid the total insulin in your system will be Novorapid + Lantus. If you drop the Lantus you will have less insulin in you, so you might need more Novorapid when you eat.
When you inject Lantus it crystallizes under your skin and slowly dissolves over 24 hours. So, say for example that you take 24 units of Lantus, which is 1 unit per hour. If you halve that, you'll only have 0.5 units per hour. If you test before and 2 hours after eating then in those 2 hours you will have 1 unit less of total insulin (2 x 0.5) than when you were on 24 units, so a unit more of Novorapid may be required. It's not quite this simple, but I just wanted to give an example. This is why it's difficult to figure out if your bolus is right, until your basal is right
It isn't for certain that you would need to adjust your Novorapid as if you are only tweaking your Lantus, it might not make much of a difference. The only way to tell is to make sure you've got your basal right, then once that is sorted you need to work on the Novorapid by looking at your pre and post-meal readings. Then once a few of those are consistent you can figure out what your insulin/carb ratio is.
Hope I've explained this right - shout out it I haven't!