Raising bed what to do with existing plants and when

I am totally new to gardening and have no idea really what I am doing. I have a very clay soil and this year have mixed in some compost and planted various plants. I decided for next year I want to raise the bed by around 6 inches (as the fence one side is good for that height). Its to early to tell how my current plants are doing but I want to keep any that live to the end of summer, I also want to plant more raspberries in November (I think thats the correct time).
So my 1st question is when is a good time to do this?
2nd Should I just dig up all the plants I want to keep and replant then as they were?
3rd What should I raise the bed with, is just compost best? & how much should I mix in the soil below or just put it ontop.
4th I get alot of slugs that have eated a huge amount of the lettuce I planted is there anything I can do while raising the bed to reduce there numbers?
The plants I currently have that I would want to keep are:
2 x Strawberry bought in pots and look to be doing and and were fairly large when bought.
1 x Tayberry 2 year potted plant planted 2 weeks ago
2 x loganberry one 1 yr potted planted 2-3 months ago 1 2 year pooted planted 2 weeks ago
1 x Black currant age unknown planted 2 weeks ago
1 x raspberry planted 3 months ago only 1 out of 4 to grow.
Any additional advice welcome. Thank you in advance.