When converting you're old legacy blocks to Dynamic blocks, you may have noticed some problems with your dynamic blocks. Especially blocks that contain existing attribute data information. The main problem happens when you try and redefine an existing block containing attributes, with a new Dynamic block with locked attributes. This causes the blocks attribute grips to show, and not be locked in all existing occurrences of the block. So you're legacy blocks attributes wont work when being redefined.
Now I've heard some people are going around erasing their old attribute blocks and re-inserting the new Dynamic block. Others are leaving the old blocks as is, and newly inserted blocks will have the dynamic properties, that work on the attributes.
An easier method is simply use the ATTSYNC command. This command allows you to SYNC or update all instances of a selected block with the current attributes defined for the block. So, it will quickly update all your old legacy blocks with un-locked attributes to locked attributes. Making them work in your dynamic blocks again. Another command you can use is the express tools BATTMAN. There is a SYNC button contained inside there.
Below is some images to better explain.
Here is an image of old legacy blocks with un-locked attributes.
This image shows existing regular blocks once redefined by a Dynamic block. Notice the Dark blue attribute grips show along with the light blue Dynamic block grips.
This shows the prompts for ATTYNC. Specify YES to process your blocks in the drawing. This process needs to be done on each different block name that you want to update.
Once ATTSYNC is completed, all your existing blocks with that block name are now updated to locked attributes and your Dynamic blocks should function normally.