Diamond tables command a higher price on the used market than do Gold Crowns.
I would say that if you can find a ProAm at $3,000 or less, it will be better to get that. $3,500 range like they used to be priced for used a few years back would make the GC a more attractive option.
Some people are stealing GC's these days, less than $1,000 from sellers in desperation.
A "Diamondizing" would cost more than $1,000 easily ....figure, that the Artemis rubber alone is over $300. The cloth is over $250 ...there's moving the table, setting it up, leveling, all the rail work, then putting the cloth on. Local guys here want $300 for moves, $300 for cloth, $300 for setup and level, $250 for rails....you get the idea. And that's without sub-rail extension, which is an involved process.
You would have to come in close to the $1,000 mark on the GC, then after spending on the "Diamondizing" you will not be very far off from what an actual used ProAm costs.
May not be worth it. ProAm has some advantages and features. It has the 1 piece slate. All the slate levelers. The better leg levelers. Superior frame. Some have the Dymondwood rails (virtually indestructible). The rails are less prone to warp with the laminate type construction. Best ball return. Pockets and surfaces do not damage or scuff up the cue. Table makes less noise when you play on it than a Gold Crown. It's way more solid in every way than the GC. The one-piece slate alone is a great feature, because you can tweak the level of the table in areas, and not have a seam to deal with or pop.
Here's how I look at it, if you shim the table thinking to get the pockets like the Diamond, it will not play like a Diamond at all. That is why the rails (wood part) has to be extended, so that the pocket facings are standard thickness. Shimmed means adding more facings, which makes them real bouncy and they play weird. Plus the angles won't be right anyway.
In order to get the job done right, you must go with a mechanic that has done this kind of work before and who has a good reputation. Preferably one of the guys who posts in the mechanic section on this forum. Do not trust some local installer to do that and end up with good results.
That said, to do the job right is going to cost some money. Get a quote and compare. Chances are, it's going to be better to just buy the Diamond. They are now selling under $3,000, in the mid to high 2K range.
Only thing I can say contrary to that is, if you think a shimmed down Gold Crown plays fine, or the pockets simply being tighter or smaller simulates a Diamond in your opinion and it the rest don't matter to you, or you don't mind or care....then it will not be as expensive of a job to get the table installed and just shimmed down. You could also skip the Artemis rubber, and go with Diamond Black rubber, which is less than $100. I know guys who are not sensitive to the differences, and a tight pocket is a tight pocket they don't care it's all the same to them. If that's how you feel about it, you won't mind a shimmed GC. I personally think heavily shimmed pockets play bad.
To each his own.