Trading down is risky. You cannot eliminate the risks, but you should try to reduce them as much as possible.
Let's say you plan to sell your own house for €600k and buy another for €400k.
1) You sell your own house first. Then you find it difficult to find another house. House prices increase and by the time you actually buy a house, you are paying €500k for it. The costs of the transactions, the redecoration and the moving eat up the €100k cash you were hoping to release.
2) You buy the other house first for €400k. Then prices fall and the market dies. You can't sell your own home. You end up paying two mortgages. (This was common place 10 years ago, but it's unlikely you would get a mortgage to buy a second house now, so it's less of an issue.)
You are adding the complication that you have a mortgage. When you sell your own house, you will pay off the mortgage. While you may be approved in principle to take out a new mortgage, lenders do, from time to time, pull the approval. So you could get stuck.
The best thing to do is to give more information.
Value of present house.
Amount of mortgage
Do you have a tracker?
Which lender?
Your age
Your gross salary
Likely price range of new house
Part of the country or description of the housing market - Are there plenty of houses to buy?
Is the house in your sole name or do you have to do a settlement with an ex?
Reason for trading down.
Brendan