diff --git a/configs/configupgrade/test-fixtures/valid/funcs-replaced/input/funcs-replaced.tf b/configs/configupgrade/test-fixtures/valid/funcs-replaced/input/funcs-replaced.tf
index 2cea2f8ba8..9f7336f697 100644
--- a/configs/configupgrade/test-fixtures/valid/funcs-replaced/input/funcs-replaced.tf
+++ b/configs/configupgrade/test-fixtures/valid/funcs-replaced/input/funcs-replaced.tf
@@ -13,4 +13,14 @@ locals {
 
   lookup_literal = "${lookup(map("a", "b"), "a")}"
   lookup_ref     = "${lookup(local.map, "a")}"
+
+  # Undocumented HIL implementation details that some users nonetheless relied on.
+  conv_bool_to_string  = "${__builtin_BoolToString(true)}"
+  conv_float_to_int    = "${__builtin_FloatToInt(1.5)}"
+  conv_float_to_string = "${__builtin_FloatToString(1.5)}"
+  conv_int_to_float    = "${__builtin_IntToFloat(1)}"
+  conv_int_to_string   = "${__builtin_IntToString(1)}"
+  conv_string_to_int   = "${__builtin_StringToInt("1")}"
+  conv_string_to_float = "${__builtin_StringToFloat("1.5")}"
+  conv_string_to_bool  = "${__builtin_StringToBool("true")}"
 }
diff --git a/configs/configupgrade/test-fixtures/valid/funcs-replaced/want/funcs-replaced.tf b/configs/configupgrade/test-fixtures/valid/funcs-replaced/want/funcs-replaced.tf
index 927ed5e47d..1664ee9b4f 100644
--- a/configs/configupgrade/test-fixtures/valid/funcs-replaced/want/funcs-replaced.tf
+++ b/configs/configupgrade/test-fixtures/valid/funcs-replaced/want/funcs-replaced.tf
@@ -31,4 +31,14 @@ locals {
     "a" = "b"
   }["a"]
   lookup_ref = local.map["a"]
+
+  # Undocumented HIL implementation details that some users nonetheless relied on.
+  conv_bool_to_string  = tostring(tobool(true))
+  conv_float_to_int    = floor(1.5)
+  conv_float_to_string = tostring(tonumber(1.5))
+  conv_int_to_float    = floor(1)
+  conv_int_to_string   = tostring(floor(1))
+  conv_string_to_int   = floor(tostring("1"))
+  conv_string_to_float = tonumber(tostring("1.5"))
+  conv_string_to_bool  = tobool(tostring("true"))
 }
diff --git a/configs/configupgrade/upgrade_expr.go b/configs/configupgrade/upgrade_expr.go
index 2cc700117d..aa4d2bcbc2 100644
--- a/configs/configupgrade/upgrade_expr.go
+++ b/configs/configupgrade/upgrade_expr.go
@@ -207,7 +207,7 @@ Value:
 		case int:
 			buf.WriteString(strconv.Itoa(tl))
 		case float64:
-			buf.WriteString(strconv.FormatFloat(tl, 'f', 64, 64))
+			buf.WriteString(strconv.FormatFloat(tl, 'f', -1, 64))
 		case bool:
 			if tl {
 				buf.WriteString("true")
@@ -398,6 +398,49 @@ Value:
 				buf.WriteByte(']')
 				break Value
 			}
+
+			// HIL used some undocumented special functions to implement certain
+			// operations, but since those were actually callable in real expressions
+			// some users inevitably depended on them, so we'll fix them up here.
+			// These each become two function calls to preserve the old behavior
+			// of implicitly converting to the source type first. Usage of these
+			// is relatively rare, so the result doesn't need to be too pretty.
+		case "__builtin_BoolToString":
+			buf.WriteString("tostring(tobool(")
+			buf.Write(argExprs[0])
+			buf.WriteString("))")
+			break Value
+		case "__builtin_FloatToString":
+			buf.WriteString("tostring(tonumber(")
+			buf.Write(argExprs[0])
+			buf.WriteString("))")
+			break Value
+		case "__builtin_IntToString":
+			buf.WriteString("tostring(floor(")
+			buf.Write(argExprs[0])
+			buf.WriteString("))")
+			break Value
+		case "__builtin_StringToInt":
+			buf.WriteString("floor(tostring(")
+			buf.Write(argExprs[0])
+			buf.WriteString("))")
+			break Value
+		case "__builtin_StringToFloat":
+			buf.WriteString("tonumber(tostring(")
+			buf.Write(argExprs[0])
+			buf.WriteString("))")
+			break Value
+		case "__builtin_StringToBool":
+			buf.WriteString("tobool(tostring(")
+			buf.Write(argExprs[0])
+			buf.WriteString("))")
+			break Value
+		case "__builtin_FloatToInt", "__builtin_IntToFloat":
+			// Since "floor" already has an implicit conversion of its argument
+			// to number, and the result is a whole number in either case,
+			// these ones are easier. (We no longer distinguish int and float
+			// as types in HCL2, even though HIL did.)
+			name = "floor"
 		}
 
 		buf.WriteString(name)